40 minutes - 1 hour
My MacBook Santa Rosa was heating and the noise of the fan was too loud.
I've change thermal paste and fan with a new one. The fan replacement wasn't necessary, the old one was in good shape. The thermal paste replacement has reduced the heating.
Do not replace the fan heaven if you think your MacBook make too much noise. Just replace thermal paste, dismount and clean your fan, remove dust as much as you can.