There's no single answer for your questions, as there is no single type of diaper or cover.
I can attest to the longevity of prefolds, swaddlebees o/s velour fitted, bummis covers, and wool covers.
I have been using my unbleached indian prefolds for 20 months. Aside from the few that have fallen victim to my MIL's overzealous bleaching, they are literally in as good of condition as when they were brand new. I wash an average of 3 times a week, so figure in the course of 20 months they've been washed at least a couple hundred times.
Our we've been using the swaddlebees for about 15 months, and besides it being ever so slightly less soft than when brand new, I don't see any real wear on it. Averages being washed 2x a week.
My bummis (super whisper wrap) covers have held up reasonably well. Although the velcro is a little fuzzy looking, it still holds the same as when they were new. These are also washed 2-3 times a week.
As for my wool covers (a few I made, an Aristocrats, a couple LanaCare, and a Babyology), I've noticed the most wear on these, only because my toddler is probably the hardest on these. As far as wear and tear, it's really only massive pilling, which I get around to shaving every so often. If she would have been wearing something over them all summer, they probably would have very little wear and tear. I am starting to notice some light staining just from being used for as long as they have, and is really more from the fact that I'm afraid to scrub them up too much because I accidentally felted a cover when I really went after the stains.