20 - 30 minuten
Battery life issues on a heavily used 2-years old 3Gs (I believe we've had about 60% of the original battery life).
Used iFixit guides - They're excellent.
Ordered 2 batteries first - 1 of them did not charge on the phones.
Had a 2nd order of 2 batteries - Again, 1 of them did not charge.
3Gs tend to be very picky about batteries (From what I've read in forums), so I ordered x2 the amount of batteries I needed.
Currently I have 2 iPhone 3Gs devices that run like new phones with great battery life.