Context
I'm starting a new D&D5e table with 6 players. As the sessions will be played during the week, mostly at night, and most of us need to be up by the morning the next day, our time is hard-limited to 4 hours. As the DM, I would like the sessions to not feel "interrupted", i.e., I would like to not finish the session inside a dungeon in a completely random place. I am a bit traumatized by Death House from CoS, which should be a One Shot and actually took us 4 sessions. (four of the 6 players on this table were in the mentioned Death House)
Additional Info
Three of the 6 players are used to D&D5e enough to keep it flowing continuously, while one is not as used to it and still pauses sometimes, mainly on combat, to wonder what she can do, and the other two have never played an RPG before, except for a one shot of CoC we had last week.
The adventure we're going to play is LMoP, with a slight different hook, which will take some extra time, because they will present their characters to me and talk about their goals and background, which would usually be made in Session 0.
The Question
How can I estimate how long an adventure will be, so I can prepare in advance some good break points where the session doesn't seem abruptly interrupted? In my specific case, how can I estimate how long will Goblin's Arrows (Chapter 1 from LMoP) will take, taking into account the experience of my players?
Further Clarification
The Goblin's Arrows, as an example, doesn't seem to have good break points by itself. Ending the session as soon as they defeat the ambush is way too early, I suppose, and if they go straight forward to the Cragmaw Hideout, there is no resting point or any moment where we could pause. How can I know that there will be enough time to finish the dungeon, and, if not, estimate where they should be by our time limit?
Note: I know that, in the end, it all depends on how my players will behave, if they are going straight forward to the goals or if they are going to take their time and this is a hard question for a sandbox adventure, but as I said, the point is to make a (not completely precise) estimation.
While the question itself is related to this one, the answers seemed to focus more on what the DM should prepare to not have a too short or too long session, while I am more interested on how can I prepare a good point to finish the session. Also my question is specific to D&D 5e and LMoP.
Best Answer
You cannot estimate how long an adventure will take.
In my experience, it is basically impossible to make any sort of precise, accurate estimate regarding how long any given adventure will take. While it may be possible to get in the ballpark (a single encounter isn't going to take 4 hours, and a huge campaign isn't going to take a single session), there will almost always be unexpected externalities that cause you to go way over or way under on your estimates.
For example, I once prepped a multi-hour encounter with a camp of bandits, only to have the PCs agree to pay off the bandits and skip the entire encounter. In another case, I've played in a game where we found a cave, said "nope," and left, and instead went to harass a random NPC for a while.
In your example, it's possible that the character introduction section could take 5 minutes ("My name is [x], I'm from [y]") or it could take the whole session. Then, the initial goblin ambush can be quite deadly, especially if you're using surprise rules--the session can end there, if your players are unlucky. As your players encounter the sentry goblins in the guard post, they could engage in a protracted combat, or bypass the encounter by sneaking through, or burn the whole thing down.
As you can see, the breadth of options that PCs have makes it very hard to estimate how long it will take for them to get through any particular section of the adventure.
All it can take for you to wildly off in your estimates is for one player to decide that they feel like doing something different at any given moment. Maybe instead of charging into battle like they have every time, the barbarian decides to be a bit more cautious or talkative, for instance.
Breakpoints are everywhere
Generally, I find it perfectly ok to have a breakpoint anywhere that's not in the middle of a combat encounter. For example, the book breaks down a number of encounters in sequence: the goblin blind, the wolves, the steep passage, and so forth. While the party's experience through the dungeon is continuous, it's really organized by different scenes. Ending the session right before or after these is a natural breakpoint, since each is a scene transition.
Also, your players might not care about "natural" breakpoints as much as you do. Speaking for myself, I'd rather have a session end when I have to go home instead of being forced to stick around for extra time just for plot reasons. I'd ask your players how much they care about natural breakpoints versus going home on time--you might be surprised at their responses.