It depends.
I would recommend that the average dungeon be designed to be completed in one session, so it would depend on how long your session is and how quickly you get through encounters. There's also a difference between number of rooms in the dungeon, number of encounters in the dungeon, and how many encounters are actually required to complete the dungeon.
For my group (3.x), we typically play for between five and seven hours, and can generally get through about five combats (plus a couple of roleplay encounters) in that time. So, I would typically build a dungeon with maybe a dozen rooms, up to eight (combat) encounters, but with two or three paths through the dungeon each taking in a subset of 5-6 of those encounters.
Obviously, if the players decide to 'clear out' the dungeon, it will take them more than a session. And, of course, I sometimes use shorter and longer dungeons (variety being the spice of life, and all that), but in general, I find that this arrangement works pretty well.
(It's perhaps also worth noting that we don't get together as often as I would like, and tend to be somewhat erratic in our scheduling. As such, I've found it a good idea to end each session at a home base, rather than in the middle of a dungeon. If we managed to play every week, I would be more comfortable using bigger dungeons. Even then, I don't think I would want to run the same dungeon for more than 2-3 sessions, though.)