20 - 30 minutos
My 3GS was out of power every 10 minutes! and shutting down himself.
I ordered an replacement Battery from somewhere else and surprise: it didn't work!
The second one did the same thing and then i ordered(I'm from Germany) one from iFixit. I should have done this before. No stupid APN comparison etc.
I just followed all the steps from the iFixit repair manual and it worked really well. The first try was the last one =)
Just order your replacement Parts at iFixit and follow the instructions and everything will be fine.