20 - 30 minutos
The original battery in my iPhone 3GS died - it could not hold a charge.
I ordered the $20.00 kit, which included the battery, screwdriver and lifting tool. I watched the 15 minute how-to video, then went to work. Things went pretty well except I broke one end of the lifting tool trying to pry out the old battery. Reconnecting some of the small cables was tricky also.
Watching the 15-minute video was the biggest help. I paused it several times, while duplicating the steps on my phone.