20 - 30 Minuten
Battery life terrible in 2-year-old 3GS. Don't want to replace until the 5 comes out.
Everything was as described in the guide but it took me a lot longer than the suggested 20-30 minutes. More like 2-2.5 hours. Part is unfamiliarity with this kind of repair, but I also had to do part of the reassembly multiple times to get the touch screen working (see below).
The connection of the display to the logic board is the most tricky part. It took me multiple time to get those reconnected properly. Pay very close attention, especially to the #3 connection wire, which has a tricky interface with the logic board. Look closely at the pictures in the guide and in other similar guides to get that right.