20 - 30 minutos
Though I upgraded to an iPhone 5, with my 6gig shared data plan I keep the 3GS as a "music machine"!
When the battery got to about two hous of life - that's when I visited iFixit.com - change was easy with all the guides...
IFix it had all the info I needed to complete the task. Even tell you what "might go wrong" and what to look for...
I ordered the tools recommended for another $9.00 or something like that, for under $25 my 3GS is holding a charge again and I'm very happy!