5 - 15 minutes
I have had my MacBook Pro for nearly 4 years now, and it continues to work well (especially after I traded in a disastrous Mountain Lion for some Mavericks), except for the fact that I could barely eek out an hour on the battery - with the screen turned down low, and opening one thing at a time. Time to give my Mac some new life.
Completely straightforward, just as the instructions on the site showed. Took me less than 10 minutes - including several to find my tool set. The one issue I had is that when I initially tried turning it back on all I got was a blank screen and it refused to boot. After a little bit of dejection and then some problem solving, I realized that the problem was actually a USB hub that I had plugged in. I unplugged it and the MacBook started right up. New battery works great!
Just follow the directions and don't stress if things seem to go sideways.