20 - 30 minutos
My iPhone 3GS started to shut down automatically some months ago. I first thought it was crashing because of the battery status often showing more than 40% of load but I was wrong. When I enabled internet access over edge or 3G, it crashed after less than 5 minutes. It never told me the battery was low, so I first reinstalled all software and did all I knew (recovery, with/without backup and so on). Then I decided it had to be the battery because the crashes never happened when the phone was attached to the battery loader. So I ordered the replacement battery!
It was really nice. The manual on iFixit was very precise I hadn't to think what to do, only follow the instructions. It was so easy, of course you have to be a little prepared in working with little electronic things, but if you pay attention on the little parts, don't use too much force and lose your parts, you're on the right path!
When it says to remove the label "Do not remove", don't be afraid of damaging it. It probably will, but that doesn't matter.
iPhone 3GS Replacement Battery
Phillips #00 Screwdriver