I agree, it might take 20-30 minutes, but at least half of that time is spent removing and replacing the firewall hatch. I think the actual time to swap the belt itself for me was 5 min or less. Super easy, but be sure you have the correct wrench (or socket) - 24mm IIRC for the tensioner pulley.
And, be sure that you have the belt w/AC. No Boxster are sold in the US w/o AC, but surprisingly, they will sell you a belt for a non-AC car which is shorter and won't fit.