4e is modeled on approximately 10 encounters/level. So if you are playing 2 encounters/session then you will level approximately once every 2.5 months.
The experience point numbers in the game are built
so that characters complete eight to ten encounters
for every level they gain. In practice, that’s six to eight
encounters, one major quest, and one minor quest per
character in the party. (DMG 121)
That means that yes it will take you a bit over 2 years to get to L10 playing twice a month and probably longer to reach higher levels as paragon and epic encounters may take longer (higher XP budgets, PC damage doesn't scale as well with monster health at epic etc)
This has been borne out in my experience. My group meets weekly for about 4 hours. We usually get through two combat encounters in a session (or 1 combat and a social encounter or similar). We have been playing for a bit over a year and are nearly at L10.
Theoretically if this math continues (I don't think it scales quite the same way) and you can continue to finish 2 encounters in a session it will take aproximately 150 sessions to get to L30. at 26 sessions/year it will take a bit over 5 and a half years to level all the way to epic.
Addendum:
Here is a breakdown of time taken to Level from 1-30 assuming 2 encounter/session 26 sessions/year at 6, 8, and 10 encounter/level:
- 10 encounter/level -> 300 encounters -> 150 sessions -> 5.76 years
- 8 encounters/level -> 240 encounters -> 120 sessions -> 4.61 years
- 6 encounters/level -> 180 encounter -> 90 sessions -> 3.46 years
This gives you an idea at least of potential different rates at which you can level depending on how challenging your adventuring becomes.
Are you familiar with the Same Page Tool? It sounds like you had expectations that you tried to convey subtly to the group in-game, but this wasn't overwhelming enough to overturn their existing expectations or the conflicting messages being sent by your campaign kickoff's dominant tropes, so you weren't on the same page.
Getting on the same page is the first step toward your players having their PCs act like real people – you can establish your expectation, and the possibility, that they can ignore the common game-like meta structures that are often taken for granted around a roleplaying game. Things like they are all a party and cooperate and they're aiming for survival of their PCs rather than character embodiment are common assumptions that are often necessary for certain kinds of campaigns, because it is easy and expedient to say that some play possibilities are "off limits" in order for the group to focus their game time on the fruitful voids of the campaign.
What appears to be the problem is that you have tacitly broadened the fruitful void, without notifying the players of this strongly enough to make them set aside their (productive, functional) RPG-playing conventions. On top of that you set up the campaign with a very standard "form party, loot dungeon" structure, which strongly conveys a standard "dungeoncrawl campaign" set of expectations that are the opposite of what you seem to have hoped for.
How to get them play their PCs like real people is then a two-step process: first, clearly grant them the breadth of allowed play possibilities necessary to be able to play them like real people. Second, cultivate a group value of character embodiment.
The second is the hardest part actually, and the how can't be covered here because the barriers to doing that are personal and depend on your players. Given that they haven't even yet become aware of the possibility with the first step, I have no data to even begin giving advice on the second.
So, that makes the first step very important: get on the same page, eliminate the assumptions about how to play a dungeoncrawl-type fantasy RPG, and replace those assumptions with explicit understanding that you're aiming for humanist drama in a fantasy context. Once you've had this conversation, only then can you even find out whether your players are interested in embodiment-focused play and what their individual barriers for that are, if any. Be aware that they may not be interested in this kind of play; be prepared to have a conversation that is about negotiating a common ground, and it may not lead to the sort of play you're looking for. It's possibly you'll all get on the same page, but if you can't, that may mean the group can't continue – but that's better than forging ahead with conflicting expectation and play goals.
Best Answer
In Situ Introductions
Also sometimes called 'in medias res', this assume that the group is already a group. You should sit down with your players, and get them to make up what kind of group they are (mercenary company, treasure hunters, powerful Knights of the Kingdom, a group of childhood friends who all pledged to become adventurers, the graduating class of a martial academie, one guy who has worked with all these other guys before, etc), and alter your beginning plot hooks to be specific to the kind of group. They should work out if the group has a name, what it is, and how they present themselves to the rest of the group 'the grim guy', 'the funny guy', 'the practical guy', 'the head-in-the-clouds wizard' etc. This is a character-building exercise. It's also a great excuse for why they're working together.
Personal Connection
Sometimes you know a guy, and that guy knows another guy, and that guy has worked with another guy, and that guy has a clingy girlfriend who refuses to be left behind and is really, really good with knives. This style of storyline starts with one character getting given a story hook, which he knows he'll need some help to roll up. Then, going around the table, the characters daisy-chain how they know the previous guy (or a different guy already in the group) and why they'd be willing to come along. It neatly puts motivation in the hands of the players without running into the 'why would I go along' problem, since they solve it explicitly.
Alright, youse mugs
Part of character creation mandates that the character must end up 'in jail' 'part of the mafia' 'in a specific diner on 9:43pm on a sunday night'. Then they get involved due to that fact. The Mob Boss makes them an offer that, as wiseguys, they can't refuse. They're let out of jail and given magical bomb collars (that are NOT plot devices and can be removed given the right mojo) that force them to act as shock troops for the Empire. Or the jail is attacked and they have to survive goblin attack on the town which by the end of it leads into more plot etc. Everyone in the diner is shiftless and at loose ends and so when a 'gal' comes in cold and wet and talking about the headless Rider, they end up drawn in. Etc.
Something about character creation puts them in a position to get hooked, and the hook is 'forceful' - refusing it is less likely than a simple job offer or request.
Piecemeal
Jim the Paladin wakes up in a jail cell and breaks out. Tim the Rogue is being carried on a stretcher towards the Incinerator. He fights the guards and frees him, and him and Tim are now a team with the goal of 'get the heck out of here'. They run into Bertrand the Ranger who has been enslaved in the kitchens in the next room, and after that they rescue a Wizard trying to fight off a [Monster] in the entryhall.
By the time they break out, they've learned the nefarious plans of the BBEG, bonded together, and all have the goal of 'revenge on that BBEG'. Not that you might not need to hook them back into the main plot if they instead decide to go start a bar together or something (BBEG burns down their bar, king offers them money to start a bar if they fight the BBEG, etc).
The Ideal First Session
The characters need to be introduced - The PCs need to have something highlight who and what they are, even as simple as a one-sentence description and a name.
Something Awesome needs to happen - If you can manage the PCs being the ones doing it, that's even better. But an Airship smashing through a castle wall, a bad guy swearing revenge, or an awesome bare-knuckle prisoner breakout/rescue scene. Something that gets people excited.
Some kind of foe needs to be introduced - Even if it's just a small starting adventure with a poor villain, Bogrob the Goblin Shaman needs to be mentioned in the first session. You can always 'move it up' to a larger foe as the story continues, but without a foe, stories with protagonists.. dwindle.
Someone needs to win something - even if it's just the favour of a barmaid. Plot exposition is important, but without a victory of some description, even one by the enemies over the PCs, people won't remember any climaxes from the first session and be less excited about the second.
The tone needs to be set - this is probably the most important. If you want Swashbuckling Heroism, swashbuckling needs to happen, so does Heroism. Ideally this is the PCs. But if not, still needs to happen. If it's Noir, there needs to be rain, damsels, chain-smoking, old-school black and white film techniques in the description, and trenchcoats. If it's dark and gritty urban fantasy, there needs to be lots of mystery and things left unexplained, horrible things happening to people or characters, etc. Tone, set, important.
Characters each need to do something that shows off their nature - saving someone, stabbing something from the shadows, stealing something, casting a powerful spell that has foes cringing in terror even before it tears them apart, so forth. You need to get a read on what kind of acts they want their character to perform, and set things up so they CAN (if they don't bite the first hook, lay a different hook - you likely read the character wrong or they missed the hook) do the thing. One per character. Important.
There needs to be a mix of action, investigation, mystery, non-combat character actions, and combat character actions - a lot of GMs use the first session to give a ton of backstory exposition, sometimes through a mouthpiece NPC. This is a really terrible idea. Could not be worse. Mix it up. Have a small fight, the PCs walk across town and interact with a baker and a merchant, they get told tantalizing hints about the plot from a guy, their contact is gone and they search through his things and find a scrap of red paper embossed with a seal, etc (keep in mind the Three Clue Rule).
How to Introduce NPCs 101
Make them people, with their own lives. Nothing sets up a character like having the PCs show up as he's wiping down his bartop, and he's not exactly keen to see them. Small details, like greasy white mutton chops, smokes a cigar, old and doesn't give a darn, wears only the finest pressed robes, bit of a ponce. Don't be afraid to give them negative qualities.
Finally, make their motivation pretty obvious. They are there for a reason, like to get paid, to get the PCs to do what they want, etc. Sure, a good liar gives no sign of that - make all your NPCs bad liars. Save the really really good liars for very rare use, when they betray the party and gloat about how the party bought into all their bull. People will get really mad, it's great.
So, in short;