20 - 30 minutos
My 3GS was slow, locking up, resetting and lasting less than an hour on battery.
Piece of cake and now my phone is good as new!
The display does take a lot of force to lift and mine popped up suddenly, pulling some cables loose. Not a huge problem, but not idea. It pays to be really careful lifting the display. The clip on cable 3 from step 6 would not lift for me and I even broke a tiny piece off. It seemed very fragile so I didn't force it. But I was able to work the cable back into place eventually.