30 minutes - 2 hours
My right fan was dead, never showed as running when I checked istat pro. Led to my mac running hotter than it should. Figured I would replace both since I already had to open it for the one.
The only complaint was UPS delivered my fans a day late when I paid for next day air. The only difficult part was the small cable going across (and attached to) the left fan. (I forget the step #) Guide was thorough and VERY helpful.
Kapton tape! The electrical tape covering the fans and vent ports were very sticky and did not want to attach to the new fans. I covered the old tape with Kapton. Also, the steps that require you to remove cables taped down the mother board need to be replaced as well.