20 - 30 minutos
After about 22 months, the battery would dry too fast. I was happy when it lasted more than 8 hours of stand-by mode (not using it!)
It went like a charm for the first part. Opening the 3GS is really easy. But when it came to actually removing the old battery, it was a bit more difficult. I don't know what kind of adhesive they use but it's quite tricky to remove it. Otherwise all went well.
The battery is firmly attached to the rear case and quite difficult to remove and the "opening tool" which comes with the spare battery is not strong enough to do the job. Pay attention not to break the rear case as well!