I've heard that zoom lenses after some intensive usage become "sticky" and difficult to operate. What's the aproximate durability for a zoom lens like a
Canon EF-S 18-135, shooting ca. 1000 - 2000 shots a month?
There is no approximate durability. It depends on many factors.
If durability is important to you, beyond proper care I would recommend a Canon L series lens that is weather sealed and likely is made of longer lasting materials. See this for much more information: What makes a camera 'weather sealed'?
This is entirely too variable to give a meaningful answer to. Higher quality lenses will last longer under the same usage criteria. But an abused high quality lens won't last as long as a well cared for cheap lens. (Although I suppose some might argue that many cheap lenses effectively come broken from the factory.)
Additionally, lenses don't just become sticky. In fact, many modes of failure are actually the opposite, a lens stops holding its focus point or focal length when angling the camera. This results in settings drifting. The lens becomes too loose.
Ideally, you want a lens that moves smoothly, but stays where you put it. Too sticky or too loose are both problems. This is why I say some would argue cheap lenses come broken from the factory because cheaper lenses don't hold focus or focal length particularly well and will go out of position if you point the camera up or down briefly.