20 - 30 minutos
After about 3 years of heavy-duty use, my iPhone 3GS battery just could not hold a charge anymore - shutting off almost instantly when unplugged.
Similar to the way the iPod's shuffle always seems to play the most appropriate songs, the phone would always shut down during conversations I would rather not be having. I mean seriously - who uses an iPhone to actually talk when there is so much else to do with it?
Since this I already used my 2 year upgrade to get my fiancé an iPhone for her birthday, the 3GS had to be revived.
After waiting over a week for USPS "Priority" to deliver my replacement battery, it only took about 40 minutes to follow the iFixit guide. And about 5 minutes of that was looking for my headband flashlight, and another 5 or so to pry out the old battery after heating with a hair dryer. That was probably the hardest part - that glue was strong and I was afraid to use too much force. Ribbon connector #3 was tricky to get back into place too. I don't know how those 8 year olds in the factory in China do it. Scraping out 3 years of hand gunk and pocket lint once I had the phone apart took a little time too. Overall, the procedure was pretty easy.
Lay off the caffeine before swapping out your battery. Shaky hands, 2.3mm screws, and tiny ribbon connectors do not mix well.
Take your time, read the directions and enjoy your revived iPhone!