>>305694587
It really is 3-5 hours long by default. I think it even says 2-4 hours per default in the map description. 
If you know where everything is and how to solve the puzzles, it's much faster in subsequent runs, though, so not being able to save is a bit less of a concern. One of us speedran it alone in 1,5 hours and said he could do it even faster. You being four players also speeds things up.