10 - 45 minutes
My battery was at less than half its original max. I couldn't work on an unplugged computer for more than an hour.
Smooth at silk. Remove screws, pop the cover, unplug old battery, plug in the new, voilà.
The computer's clock will most likely reset once you put he new battery in, but that's about the only thing I found I had to fix post-installation.
Oh, and it was a tad dusty inside, so some canned air might be good.