Fifteen Minutes

Excellent game! As others have said, it's well worth challenging yourself to complete it without the walkthrough. Word of advice, though - do NOT save after answering any questions UNTIL you've been given the okay by the principal!! Needless repetitive hell when you realize you've messed that up. (Otherwise, proud to say I finished in a matter of hours, though I did have to check the walkthrough after somehow failing to examine the rope :P)

As someone else said, the clock tick is really inconsistent, so undo is your best friend here. Above all, it's not as hard as it seems, so long as you keep notes on the offset from current time for each version of yourself.