30 minutos - 2 horas
The rear camera on my iPhone 4S was partly out of focus, and the autofocus mechanics did not work at all.
I followed the description given by iFixit, and I felt the process went smoothly. It took me about 30-45 minutes to complete the whole job.
The guide: iPhone 4S Rear Camera Replacement
I would recommend to use a "screw chart" to have a place to "store" the disassembled parts and screws. This can save you a lot of time when you reassemble the phone.