2 - 4 Stunden
I wanted to have my Time Machine backup drive inside of the Apple Mac Mini. And, I wanted a bigger main drive too.
The upgrade went far easier than I thought it would. I've replaced hard drives and fans in older Macs before thanks to iFixit.
The great thing is that your instructions and videos have improved dramatically over the years (and they weren't bad to start with!) so serious upgrades should not be feared!
Read through the instructions (or watch the video) completely before starting the project. That way you'll know what to expect as you start the project.