There are about 300 doors per dungeon, but the game only counts doors you opened directly (IE before you pick up the crystal). This is easy to verify by starting a new game and looking at the number of doors displayed in the UI at the bottom right just before you pick up the crystal, then either dying or carrying the crystal to the exit. Additional doors opened while the crystal is being carried do not count toward your total. This means that using the average completion time for a normal run as a proxy for the number of hours required to get credit for opening 10,000 doors is flawed. If we assume people open on average 66-75% of doors on a given floor, and consider that the time per door generally goes up as you open more doors (since your ratio of powered to unpowered rooms declines and you're therefore more likely to encounter monsters) we can assess that the actual completion time for this achievement is significantly higher than 100 hours.