Another consideration is what TYPE of haunt it is. The ones with great eye-candy, you want to last and last and last, so you can enjoy looking around. The ones that are less family friendly and put the emphasis on really scaring the piss out of patrons are a different kettle of fish, however. Many people enter thinking they want it to last and last, but after the first few minutes, they want it to END.
Ours takes about 40 minutes to go through if you don't power walk. Most all say that it was much longer than they expected, and a good deal of them admit that they couldn't wait to get out (and some always exit early). But they also say they had a great time. So it's a double edged sword: they love the length before and after being in it, but while they're actually THERE, they're stressing to get out!
This would also explain why most everyone is out of breath at the end, even though they are not allowed to run. It's the adrenaline and accelerated heart rate.