20 - 30 minutos
I had an aging iPhone 3GS - whose battery life had reached maybe an hour or three after a full charge. I had passed it on to my daughter - who mostly used it as an iPod - but a recent upgrade to iO6 had left it behaving erratically - like switching itself off after 2 minutes when still showing 80% charge.
The iFixit forums seemed to suggest that a battery swap out might be the answer ...
It went ...
Very Well :-)
I had printed of the guide from the site , and using the kit that came with the battery I slowly (very slowly) carried out the swap.
It's nothing new, but I'd say draw a little map of the device, note where the screws came out and in what sequence and take your time.
Oh, and if at any point it seems like you're needing to use a little to much force to pry something apart - take a beat and check you've disconnected everything you should have :-)