20 - 30 minutos
Two hand me down phones (from parents to kids) when parents upgraded to 4S. The batteries in the 3GSs were getting weak after two years.
No problems. Very clear step by step direction make it a snap. All you need is patience with small parts.
Have a small container to put screws in. Dropping one on the carpet (it would be very difficult to find) could really derail this project.