Usually the games askyou to use a three-hours timer and the players need to act fast in order to end the dungeon before time is off. This won't work in a play by forum: how do you measure it? Number of posts? The closest thing I can use is number of rounds.
At the table, saying "we search the whole wall" takes almost no time. It would be fine if it took the same number of rounds in the play by forum, but it's going to change wildly as players decide to searche all the wrong walls before checking the right one. It is an unfair loss of precious, scarce "time"