Dungeon Design

Some thoughts have crossed my mind related to dungeon design. Do you think that, in a classic dungeon-crawl type scenario, that a dungeon should have about the number of encounters required to gain a character level? Such that if PC's enter a dungeon as level 1 PC's, right about the same time they mostly clear the first level of the dungeon, they gain a character level? If so, that would seem to suggest that the average dungeon level should have 12-15 encounters, of which some are very challenging, and some are less than challenging. While I'm on that subject, how long do your players take per encounter in real game time? Meaning, in my campaigns each encounter takes roughly 2 hours of game time, which seems like a lot. We do generally follow a timer rule in order to keep things moving but just curious if my 2 hour experience is that same with other groups.