5 - 15 minutes
I was getting a "Battery Condition: Replace Now" warning, so I decided to replace the battery with a new one.
Battery was extremely easy to replace. I didn't time myself, but I imagine the 10-15 minute estimate is accurate. I had never opened my Macbook Pro before this repair, so it was neat to see the internals of the computer.
Make sure to get the right tools (tri wing screwdriver is hardest to acquire, so order it along with the battery from ifixit). Be sure to keep the screws separated and in a safe place.