1 - 2 hours
My 24" iMac 2.16 GHZ (late 2006) was experiencing power problems. It would take two or three presses of the power button to start up. Then one day it wouldn't boot up at all.
The directions were first rate. I replaced the power supply in just under an hour. This repair saved me the $1500 I was going to spend to get a new Mac.
When you re-install the display, make sure the cables for the iSight camera and microphone are sticking out so they don't get stuck under the display. Also, you need a T6 Torx wrench for this job, which is not listed in the "required tools"section.