20 - 30 Minuten
My wife's iPhone 3GS wouldn't make it half a day without loosing a full charge.
I had already changed my iPhone 4 battery and it was fairly simple, but I knew the 3GS would be more tricky. After watching the iFixit videos, I thought I would give it a shot. 20 minutes after I started, new battery installed! That was three days ago and we still have recharged it!
Don't worry, go for it!