20 - 30 Minuten
My 2.5 year old iPhone 3GS began overheating, dying and shutting itself off recently. I figured the battery was the problem and planed to replace it. It died one morning and wouldn't power on at all, so I bought an iPhone 4S. I've never worked on an iphone but figured I'd try anyway, that way I could sell or give my 32gb 3GS to someone. It had only a small scratch on the screen and worked great.
It was much easier than expected. a friend helped me as I'm not great with tiny screws. It took about 45 minutes. I rushed lifting the screen and popped connectors 1, 2, and 3 out of their holes, they were hard getting back in having not seen their original position. It was an easy experience overall.
To save money after having just bought an iphone 3s, I only bought the spudger and suction cup hoping I'd have a 00 screwdriver. The battery came with the screwdriver so I saved money there, and it came with a spudger like tool I probably could've saved on. I let the battery drain and recharged it to 100% to re calibrate it. Also, I should've taken photos of the process. The guide here was pretty helpful. I couldn't have done it otherwise.