5 - 30 minutes
After about five years as my main system, my Macbook Pro was grinding and beachballing and taking hours to do five minute tasks. I'd also lost use of the disk drive. The shops all wrote it off as "cheaper to buy a new one", but I was pretty sure they were wrong.
It was honestly pretty uneventful! The instructions were thorough, the kits were complete. It took about half an hour. The longest part was waiting for my files to transfer from my external drive.
Worth it - do it!