20 - 30 minutos
My 3GS would shut down when the battery showed 80%. This happened pretty suddenly. At first it would shutdown at 20%, then 60%, and within a week or two 80%. Also, we had dropped the phone and cracked the glass.
The battery replacement gets pretty easy with practice. We had replaced 2 other batteries, so this was a breeze. The glass replacement was a little more difficult. Getting the glass seperated from the plastic frame was pretty difficult. In the end it was successful.
If you're just going to buy the glass, make sure to buy the adhesive too. If I had it to do over again, I'd buy the whole glass/ LCD assembly and skip the hair dryer.