20 - 30 minutos
My 3Gs would completely die when it reached around 40% battery power.
It was easy enough. The guide and video helped out a whole lot. It took me about 40 minutes to finish, only because it was my first time. Now that I know how to do it, I could probably finish in half the time.
Don't bother sending your product to apple, unless you really have to. I would recommend attempting to fix the problem yourself. It's not very hard and could end up saving you a lot of money and time like it did for me.