5 - 30 minuten
My wife got the dreaded gray screen and her Macbook Pro wouldn't boot. I figured it must be the hard drive so I while doing some research online I diagnosed the issue using the ifixit.com answers section. I then read they sell repair kits so I thought I would give it a shot based on the success from previous users.
Actual HD replacement was very easy. The iFixit bit drive kit was amazing and a great value. The instructions were very clear and easy to follow. I had a more difficulty reinstalling the OS since my wife, of course, didn't keep her recovery discs. Luckily, I have a Macbook Air and was able to download Mountain Lion from the App store and created a bootable USB drive to reinstall OS X on her laptop. We have time capsule so using Migration Assistant to get back all of her data was a breeze.
If you own a Mac you should use Time Capsule or Time Machine to backup your data. If you store data on your laptop that you want to keep you must back it up otherwise it is just a matter of time before the HD fails. The applies to PC or Mac. Online backup systems are okay if it is less the 50GB. Anything over that and it will take you more then a month to redownload it on your restored device or your ISP will throttle your connection for excessive data consumption.