15 minutes - 1 hour
We were running out of disk space with our 500 GB disk on our late-2009 iMac, and I was fed up managing content on external USB drives.
It was actually quite easy. It only took about 45 minutes and my 12 year old daughter did most of the work. The guide for the EMC 2308 was spot on for my iMac, which is an EMC 2551. Recovering the machine from the Time Machine backup on a USB drive worked fine, although it did take 4 hours to copy the nearly 500 GB of data over USB 2.0. We also took advantage to take out and take apart the DVD super drive to clean the lens (it stopped working years ago).
Make sure to make the last Time Machine backup before shutting down!
After the repair, when the iMac is first restarted with the new disk, the disk repair utility comes up but the new disk was not visible. Before you can restore the data from the backup, you have use the Disk Utility to format (or reformat) the new disk to the Mac Journaled FS using the "first aid" utility. Then, the new disk shows up as one of the disks that can be used as the destination for recovery.
There was one minor hiccup. Although everything was installed properly after the recovery from the backup, MS Office asked for the license key when I tried to use it. I had to dig it out of my email archives. So keep such software keys handy because they are apparently associated to the HW Id of the disk somehow.