20 - 30 minutos
My 3GS kept shutting off on me no matter how much battery life i had left. it even shut off on me when it was at 100%. so i did some research and found out that it might be the batter or it might be issues with the firmware. so i took a risk and decided to replace the battery.
it went pretty good. i was a lil sceptic at first but once i follow your guild everything went smooth and i was able to replace the battery and get the phone up and running with out any issues. and my risk paid off cuz now everything is back to normal :)
just follow all the instructions to the dot and u should be able to replace the batter with out any issues. it took me about 30mins to do it because it was my first time but it wsn't too back or too hard.