20 - 30 minuten
My battered 3GS was struggling to last a day on battery without a recharge so it was time to fixit. I initially bought a battery from a chinese site and found myself stuck in a perpetual loop of boot to home screen, then 30 secs later reboot. And on, and on, and on. Restore would not fix it.
Buyer beware: turns out that the iPhone is fussy about the part number of your replacement battery. Who would have thought?!!!
Despite saying it was good for 3GS it seems that my replacement wasn't. So...iFixit to the rescue. Make sure you get the correct APN number on your battery or you'll end up in 90 second boot loop purgatory.
My iFixit replacement initially had me even more concerned despite following the excellent repair guide. This time it didn't even get to the home screen. Just boot, boot, boot.
Eventually it was Google to the rescue: It turns out that taking the sim card out would let it boot normally and the sim card could be re-inserted after that. It has worked normally ever since. Thanks iFixt!
Make sure you get a replacement battery with the correct APN (a part number printed on the battery) to match your phone. Try to avoid destroying your battery when removing it just in case - sliding the guitar pick supplied with the tool pack under the battery in various places to weaken the adhesive before you pull it out will help there.
If you still get stuck in boot loop !&&* try removing the sim card, doing a hard reset and see if that fixes it. Failing that, Google is your friend.