Yepyep, here we go. Obviously, not all spells have duration caps, so those aren't included! Some are also nice and difficult and don't give durations (BW/BoT) :P

Courage - 13 self, 38 group
RW - 21
SR - 21
AS - 30
MO - 30*
DA - 30 (I believe the best single cast is 26 or 28, I forget).
HW - ??? (I'll guess and say its 30)
Clarity - 60

* I'm guessing it's 30, but I'm pretty sure. A prep of 50 gives 23 minutes, and the spell caps at 60. So... ;)