Good answers above, but there are so many factors involved that you cannot easily estimate this until you have experience running things for this group with this particular system.
For my 3.5 group I used to reckon I needed about an hour for a solid encounter, so expect to manage two encounters per session max. Exploration, description and roleplaying encounters can add a lot to that.
The question behind this question is... if they are having a good time, does it matter how fast they consume your material?
If they take three sessions to use the stuff you prepared for one session then you have no problem as far as I can see. If they are burning through your prepared material much faster than you expected then you have a problem.
To handle that you can
- Tweak progress in-game by taking more or less time with exploration and descriptions and allowing more or fewer breaks and social time. That takes some experience... so it does not help you much right now.
- Expect to mess up some times and say sorry. "Wow guys, you really made amazing progress... so we need to end the session for this time and I will have more for you next time."
- Prepare for it. Have about twice as much prep as you could ever expect to use: it does not go bad if it has to wait around a while.
- Hone your improv skills so that you are comfortable adding extra encounters on the fly.
- Add side-quests You can have optional stuff, one-shot adventures with no impact on your campaign, ready to drop in to any session. I find it helps to have a few typical monsters and NPCs on index cards that you can just pull out as needed.
Good luck and have fun!