5 - 15 minutes
My 27" iMac is five years old and the original 1 TB hard drive began to fail and needed to be replaced.
The replacement took less than two hours. I was very surpised at how well it went. The IFixit free instructions and tools that I purchased from them were the best and I could not have done it without them. The new 1 TB SSD Hybrid is quiet and faster than the old one. I did have some problems with the recovery from Time Machine, but got er done.
Don't be afraid to do the work on you iMac. Follow the IFixit instructions carefully.