20 - 30 minutos
My old iPhone 3GS that I passed down to my son developed battery problems, so decided to replace the battery myself and save some bucks.
The ifixit guide was great, and MJs video was awesome too. I did take my time...it probably took me almost an hour, but thats just me as I'm kinda slow... My biggest problem was getting the old battery out, as it was stuck in there very securely. I broke one end off my spudger, so I used a flat bladed screw driver to pry it out.
Take your time, follow the directions...and be sure to watch MJs excellent video several times. Don't break your spudger trying to pry the battery out, just use a screwdriver if it puts up a fight.