I think you should charge on the quality of your show other than the time it takes to walk through. We have 5 haunts which could take any where between 20 to 30 minutes to walk through but there's 5 there. Each one with an exit shooting back to our lobby where you can purchase drinks, clothes, relax and watch our big screens...and we only charge $20. Its perfect for the target market and they still have enough money to spend on refreshments and such!
I agree with the "if its too long it seems to just drag on" quote. You need a break in there some where to keep you more interested in the next scene instead of tired wanting to sit down.