20 - 30 minuten
My wife and I received two 3gs's that were fairly old and they started to randomly shut off and the back glass was cracking for some reason. After my screen popped out I realized what the problem was. So I ordered some replacement batteries.
It took me roughly 45 minutes to remove both batteries and probably 20-30 to put them both back together. It is a very simple repair, but you need to be a little gentle.
I would definitely recommend waiting for the spudger's to come in unless you have a good alternative. A credit card like device did not work very well. Also highly recommend a magnetic screwdriver like the one that comes with the iFixit replacement battery. I ended up losing one of the screws as a result of a none magnetic screwdriver. Lastly make sure you are confident that the camera is properly connected. As it's the only connector on the bottom of the logic board it slipped out the first time I put the phone together and I had to disassemble and reassemble one of the phones a second time just to fix the camera.