20 - 30 minutos
After 2 1/2 years, my 3GS battery would die afte about 12 hours.
Unfortunately, I did not double check when Google directed me to purchase the battery so I purchased the 3G battery. It did not work properly. I uninstalled and reinstalled the battery twice to make sure I didn't miss a connection. I spoke with support and they worked through it with me and we got the right battery. I can replace the battery in under 5 mins now.
Make sure you get the right battery for the phone. Put your model in the Ifixit webpage and let Ifixit direct you to the right one.