30 мин. - 2 часов
The 1TB hard drive gave a SMART warning through Tech Tools Pro. So taking no chances I used Bombich Carbon Copy Cloner to make a backup and restore partition. This was a good idea as I would have to make a Fusion Drive as the iMac originally had the 1TB Fusion using terminal in the restore partition. I the used the restore to do a clean install of OS X and migration from the clone.
The most time consuming is breaking the adhesive. My advice is take your time. You will be reusing the circular cutting wheel and "old " credit cards to break it free. I chose to loosen the right speaker and move it to expose the two hard drive bracket screws and lift the hard drive out. Putting the new hard drive back in took less than 5 minutes.
The only time you need an assistant is putting the display back for alignment; I do not think I could have done that by myself. Also, format the drive outside with a dock and run a utility to check it out before installing. Especially if the drive has bad block or was DOA! Not good. My drive was okay. Make sure your clone of your drive is bootable.