20 - 30 Minuten
My old iPhone 3GS (32GB) started powering down randomly and rebooting. When I tried to restore it, I kept getting Error 29 and the restore failed. After researching, this was indicated as a potential battery issue.
Fine, the tools and guides were great. Everything worked smoothly. The repair took me about 45 or 50 minutes total. You have to be careful of the ribbon connectors.
Make sure you have a suction cup handy-the kit does not come with one, which wasn't clear to me. I luckily had one around, and it is necessary to get the screen out.