20 - 30 minutos
My iPhone 3GS is about 2 years old. I use that thing ALL THE LIVE LONG DAY. The battery was only lasting a half day or so. An IT tech support buddy at my work said to buy a replacement battery from ifixit, since they sent the tools and provided video and pics on how to do it.
The excitement in the air was palpable. I took apart my phone, following the directions to the letter. The cute girl in the demo made it hard to focus on the phone, but I figured it out. I stopped halfway to help put the kids away. My wife saw the phone in pieces and said, "You know what you're doin'?" The doubt was in her mind. I am now the owner of my same phone, that lasts all day again.
I would have read the instructions the whole way through, at least once. That would have help out a bit, I'm sure of it.