30 minutes - 2 hours
I bought my iMac with only the platter drive because the SSD options were so expensive. After dealing with the slow drive for a while, I found a 256 gb PCIe hard drive on eBay.
The guide was spot on. The only hard part of this repair is the screen removal. The directions are clear though, and the removal went perfect. The rest of the steps are basic, and anyone could do them. In fact, anyone could probably remove the screen. It is just stressful because it feels fragile and it is an expensive piece of electronics.
I didn't buy the foam iMac pad to hold up the enclosure with the screen removed. I just laid it down on a soft surface. It worked just fine.
TR10 Torx Security Screwdriver
TR9 Torx Security Screwdriver
iFixit 4 mm Screwdriver Bit
iMac Intel 27" (2012-2015) Adhesive Strips