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)
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.
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?
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.