10 - 20 minutes
4-year old battery no longer holding a charge for more than 1-1.5 hour.
Overall very easy. The guide is very helpful and high quality tools ordered from iFixit helped tremendously.
The only tough part was reconnecting the battery terminal once the new battery is in place. The cables were a bit rigid, so it was tough to line it up perfectly. Helped to lift the battery out slightly, then connect the cable and place the battery in the slot where it belonged.