20 - 30 Minuten
Noticed one day that there were some strange marks appearing through the screen of my 3GS. The next day, the glass was bulging at the sides, and the phone would not stay on... I realised that the battery was expanding and removed it using this guide. When I unscrewed the logic board, the whole thing was bulging so much I thought it was a gonner. Now I needed a new battery.
Ordered the replacement kit from iFixit and the new part was quite easy to fit - only difficult part was that the tiny coax connector for the 3G antenna was slightly distorted (probably why the phone has always had an iffy 3G performance) so it was very difficult to click back into place. It worked!
If you notice the battery performance degrading or any strange dark zones showing on the screen - REPLACE the battery IMMEDIATELY!
A few weeks before, I'd made the mistake of letting the battery run flat and leaving it for a few days then fully charging it, which is apparently just about the worst thing you can do with a Li-ion battery! With a unit already 2 years old it was the final straw.