It's totally dependant on the car and the level of install you want done. I can do an IHF1000 in about 35 minutes... but I wouldnt do that in my car.
The wiring is general the easy part... its the routing of the mic, speaker (if used) and controller plus securing the brain and making it all tidy under the dash somewhere. Those things take time to do it damage free - two to three hours is not unreasonable.