1 - 2 horas
The hard drive failed on an older 24" iMac we use as a backup for the home office. After leaving the machine at the Apple store for repair, the store said they couldn't do the job because it appeared the computer had been worked on by an "other than Apple authorized" person. So after two 45 minute trips to Apple, I was back where I started. Apple had run their diagnostics and it appeared the hard drive was the only issue. I thought if these pinheads at the Apple store can do it, it can't be that difficult.
Even though ifixit didn't have the repair manual for the exact model iMac, it was close enough that all went well. I put in a 1TB hard drive for less than half of the repair estimate from Apple (for 500GB). The computer is up and running with the latest OSX and applications with no issues. New HD is silent and fast.
It is not extremely difficult. Be sure you have the proper tools and room to work. Stay organized. Follow the instructions exactly. Be careful with the small plugs (temperature sensor, camera, microphone) as they are delicate. Have the correct solution and cleaning cloth ready to clean the monitor and glass before reassembly. Have fun.