5 - 20 minutes
My iMac early 2006 only had an Intel Core Duo processor, I needed a 64bit processor so that I could run Mountain Lion. I discovered that I could install am Intel Core 2 Duo with a faster clock in this box.
The repair went well and ifixit tools really helped. The biggest job is pulling the chassis apart and removing the motherboard and heatsink. Again ifixit guides were critical. Once the motherboard was out, replacing the CPU was easy. Clean up the old heatsink and then a little thermal paste and it can then be reassembled.
Installing Mountain Lion is no trivial task, but once I got it working the computer was now usable with iCloud, Messages and App Store updates. I was very excited to get it upgraded.