It depends

It heavily depends on the number of players and enemies (a CR +0 encounter for 4 characters takes significantly less than a CR +0 encounter for a party of 8, and a CR +0 encounter for the same party could be composed of many weaker monsters or a single boss), on the availability of save-or-die spells and whether they land or not, on the complexity of rules involved in your PCs and NPCs (sometimes, finding the rules to grapple in D&D 3.5 greatly slowed the game. I'm pretty sure there are equivalent situations in Pathfinder too), on the players being attentive or not and, on the increasing number of options for high level monsters and PCs and, in general, on lots of independent factors that's impossible to predict in advance.

(I had gaming sessions with 5 encounters easily solved in the same 3-hour game night and in the following encounter the same time was used to play the first turn and a half of a combat, just because of a silence spell forcing the casters to be creative.)

I've come to the conclusion it's downright impossible to pre-determine the duration of an encounter before playing it, unless you fiddle with monster's HP on the run, having your enemies last longer or die faster as needed... which is probably not the game experience you were looking for and feels a little like cheating them to me, not letting their expertise or luck shorten an encounter and ultimately preventing theem from actually influencing the outputs of (that part of) your narration.

However, I feel there still is something you can do to prevent that feeling of sloppiness.
Have your encounters be meaningful. No more filler encounters, random goblins in a side-room. If you don't want to skip that room because it makes sense in the dungeon, consider having the monsters in there really weak and skip the actual combato, or even better offer your players to spend some resources ("...You lose 30 hp and 3 levels of spells and you win. Is this ok or do you want to play it?")

This means shorter dungeons and less time spent in useless goblin-mashing, which should make dungeons more interesting.
On this page, the dungeon's lenght should be tied to what the dungeon is in the game world (are they exploring some dwarven mines? That could be huge) but if you want to go for a less realistic but more gamey perspective, a dungeon should be clearable without going to rest. This usually means 4-6 encounters in my book, to be played over 2 or 3 gaming sessions. Your Mileage May Vary.
Another option is googling for some 5-room-dungeons. These are a series of 5 "rooms" (true, they are called dungeons, but the same structure is often used to describe any string of different places) following an identical structure:

• Room One: Entrance And Guardian
• Room Two: Puzzle Or Roleplaying Challenge
• Room Three: Trick or Setback
• Room Four: Climax, Big Battle Or Conflict
• Room Five: Reward, Revelation, Plot Twist

Five rooms, one to three combat encounters (in rooms 1, 3 and 4) feels like a popular choice.

Suffocation:

A character who has no air to breathe can hold her breath for 2 rounds per point of Constitution. If a character takes a standard or full-round action, the remaining duration that the character can hold her breath is reduced by 1 round. After this period of time, the character must make a DC 10 Constitution check in order to continue holding her breath. The check must be repeated each round, with the DC increasing by +1 for each previous success.

When the character fails one of these Constitution checks, she begins to suffocate. In the first round, she falls unconscious (0 hit points). In the following round, she drops to –1 hit points and is dying. In the third round, she suffocates.

Although the medium he breathes has changed, a victim of aboleth's lung is going to suffer the same fate unless he reaches water.

An Aside: On the Cruel Nature of the Spell Aboleth's Lung
The spell aboleth's lung is a 2nd-level, slow save-or-die touch spell versus foes without dispel magic and who breathe. That's... actually a lot of folks, especially at low levels. That the creature dies faster if it tries to murder the caster is tasty gravy.

The problem with the spell is when it's used versus the PCs. It's entirely possible for PCs to face foes who can employ the spell when the PCs are among the folks vulnerable to it, making it just a slow, unpleasant death sentence. Such spells are unusually unfun, making players feel helpless while also killing their characters. Usually, it's more fun--as the DM or the player--to just cast scorching ray at a dude than to slowly suffocate him while making him do math.

There's an argument that adventurers should be carrying water when adventuring, giving them the means to save their air-drowning friend. But this assumes someone trained in the skill Spellcraft successfully identifies the spell when it was cast. (Pathfinder removed from the Spellcraft skill the ability of D&D 3.5's Spellcraft skill to identify a spell that’s already in place.) If the Spellcraft roll's failed, the DM just describes the spell's effects--suffocation. Uninformed PCs are then supposed to make a leap of cartoon logic that shoving their friend's head underwater will save him, which makes about as much sense as stopping bleeding by sticking a sword in the wound.

The spell, without DM's permission, is supposed to be exclusive to gillmen. Encourage your players to enter into a gentleman's agreement wherein this spell's just used by gillmen... then don't use gillmen. The spell aboleth's lung is a war crime.