Labeling and photographing everything, then disassembling the wiring, cab interior and HVAC, steering, hydraulic controls, etc, then reassembling it, troubleshooting whatever gremlins show up and fixing them. On top of that it is a job the mechanic has never done on this tractor. I think that 20 hours is light if anything. I would quote it out at 35-40 to protect myself, and bill less if I finished early. Getting a cab from EU or Asia is a risk that the dealer is taking because there is no return policy if its wrong.
If its a fully furnished replacement cab then 20 hours sounds closer to being right.
20 hours on a clutch job seems very high. Most tractors that cab can be supported and the front rolled away for a clutch job. Replacing a cab is not at all comparable to a clutch job as the amount of work is significantly more. I would expect 6-8 hours for a clutch job.
I once took 8 hours removing and installing a wiring harness on a skidsteer that had burnt. The stupid little things catch you. Had to lift the cab with a crane to run it correctly as the manufacture installed harness prior to cab being on the machine. The fuel solenoid wire was made incorrectly, every switch wire was the same color and had to be hand traced, the harness was missing two wires that I had to add in and a few other little issues. I had figured that the job would be 3-4 hours. This was an $2,000 OEM harness that should have plugged right in, or so I was told, glad I was paid by the hour and the boss was very understanding.