20 - 30 minutes
My wife's iPhone 3GS needed a new battery. The phone needed to be recharged several times a day, and would frequently just run out of juice in the middle of calls.
The repair went exactly as described on the iFixIt website. All tools worked flawlessly and I had no issues whatsoever.
Be sure to follow the iFixIt instructions exactly as shown. When you take the screws out, lay them out in the order in which they were removed. The iPhone screws, although almost microscopic, are different sizes.