20 - 30 minuten
The problem was that my iphone 3gs was shutting when i was using any application and when i was starting it again it was starting without any pin request and with a lot less of battery.
The process was long because i live in italy so i had to wait for the new battery.... when it arrived i opened my iphone following the iFixit guide without big problems i replaced the battery and started my iphone.... Boom my iphone started to reeboot randomly every two minute. I decide to make a restore.... Boom error 29 on every try! So at this point i started to try various firmware (putting the old battery to restore and trying the new battery) and i discovered that with 3.1.3 the phone was not rebooting but i couldn't update or restore firmware (because of the 29 error). At this point i wrote to iFixit telling my story and they answered my back that they apologize for the situation and another battery is arriving to me... When the new battery arrived i opened my mobile put it inside and... BOOM it was working like a charm!!! So Thanks again iFixit!!!!!!!!
I have to suggest a thing about the steps after the 10 because i had problem width the ribbon cable labeled "7". the first time it was hard but after i discovered the trick.When you try to remove the logic board in step 14 the ribbon cable will stop you so bend it gently in the direction of the dock connector and pull out the logic board. When you will put it back there will be no trouble just try to keep the cable on the external side and the gold hook will slide without problems.