20 - 30 minutos
My iPhone 3GS wouldn't even last 20 minutes. Any and all applications made the phone turn off and it wouldn't turn back on until I plugged it in.
The repair went well. The repair manual videos were really helpful and easy to follow. The only obstacle that I had was the battery was really glued into the phone, that was the only reason the replacement took a long time. I found another review that had suggested using a hair dryer to loosen the glue on the battery and it worked! (I just wished I had seen that from the beginning). Once that obstacle was overcome, it was smooth sailing all the way!
Really order the extra spudger, the one that comes with the kit doesn't seem as sturdy. Also get the suction cup - it makes it a lot easier to get the screen off without prying anything up. I would also recommend having a hair dryer handy in case the battery is really glued in to avoid bending the battery.