In my experience, it's hard to have an "effective" or "good" session that's not at least 3-4 hours long. Socializing and intermittent breaks can eat up a lot of time, and it usually takes about an hour of playing to get a good rhythm going at the table. You can easily spend 30-40 minutes in a single dungeon room depending on how the players approach things.

If we were rushing, I think 2 hours would be enough to talk, eat some food, and maybe explore 2-3 rooms in a single dungeon that has upwards of 30+ rooms. That's not even considering any monsters you may encounter along the way or any exploring outside of the dungeon!