10 - 15 minutes
I dropped my phone and the home button stopped working...poor battery charging practices apparently were messing up my battery as well so I swapped both.
Everything went smoothly...don't use the pry tool to remove the battery though, use the clear plastic tab Apple already has supplied in the phone...otherwise there's a risk of cracking a board.
Not as scary as it looks, you really need to use some muscle to get the glass open at the bottom. Use the battery pull tab inside the phone instead of the pry tool to get the battery out safely.