As RickB stated, guns carried in kydex holsters usually have a few hot spots of wear, with basically no wear elsewhere. I suppose the timing of it depends on the guns finish.
My experience with leather has been that it wears the guns finish more overall, and leaves it looking worn, washed out and faded over time. You also have the rust issue if you sweat a lot, one thing kydex pretty much eliminates.
This is my old Colt Commander. It started out as a nice Colt blue, and lasted about a year in a couple of Galco Royal Guards before rust set in and it needed to be refinished. What you see here is the hard chrome refinish after 10+ years of hard daily use in a Blade Tech kydex IWB. The "black" streaks are actually polished chrome, and not black. They look like a mirror in person. They didnt appear overnight, more like over a couple of years. The kydex and HC knocked down 99% of the rust issue, but I still got a little "powder" like rust at and under the grip panel on the body side of the gun where it was exposed to my body now and then.
No matter what you choose, if you use and carry your gun, its going to get beat up. You might as well accept that right off. If you cant live with that, then dont carry it, and find something you arent as worried about.