1 - 3 hours
After upgrading my MacBook Pro to have an SSD I really felt how slow my iMac was, so I decided to put in the addition.
Flawlessly! The repair took around 1.5 hours. The scariest bit for me was initially taking off the glass; I was terrified it was going to shatter! Luckily it didn't.
Watch the instructional video a couple (hundred) times before attempting the install. Make sure you know what you're getting yourself into as it is not a simple upgrade.