I use my computer for 2-32 hours a day. EDIT: 2-3 hours XD
Standby: 14-16W, depending on how long I use it for (not per hour, overall)
Shut down: 15W
On average for me (a mac user :) ) it is far easier to keep it on.
PCs, on the other hand, are awful at standby. And shut down, for that...