How quickly do they wear?
Not very if you use a few precautions.
I tighten the front ring of the grinder down just fingertip snug and back it off about 1/16 to 1/8 turn. That keeps the grinder blade from wearing or scoring the plate due to being too tight. A looser ring leads to faster connective tissue build up between the blade and plate. With a bit of experimenting you'll find what works for you.
I also swipe a wee bit of Crisco or some such on the plate when I first install it on a clean grinder head to help keep it from being scraped on the blade before the meat and fat can get to it to lube it. I don't worry about it if I'm changing plate sizes and the grinder head is full of meat.
I find that if I change plates without cleaning the grinder head such as when I grind fatty stuff to a different size than lean I snug the ring down, bump the switch for a second or two and re-snug the ring. If I don't snug the ring the second time there is usually a lot of wobble in the blade and plate fit.
If I grind anything really lean like chicken and the recipe can stand it I toss the chunks with a bit of veggie oil before grinding to protect the blade and plate. That is a primarily due to a caution from grinders maker about really lean but as it's a commercial unit intended for a lot more use than occasional household use I may be pole vaulting over a mouse turd here.
I don't let the blades roll around in the sink and get dinged up. I wash them by themselves. After they are positively dry I fold them up in a paper towel so they aren't rubbing and put each in a small zippie bag to keep them from getting dinged in the box of grinder accessories. When I get a knife for each plate I intend to fold them both up together with towel between them so they don't rub and then bag them.
I hone the blades and clean up the plates on a fairly large flat diamond knife sharpener. They really haven't needed it as much as it is my early cabinetmaker's roots showing.
I guess one way to describe my thinking is that I try to treat them as I would any other sharp knife in the kitchen. I figure it is a lot easier to keep them sharp than to sharpen them if they get outright dull.
Right now I have one carbon steel straight blade and one stainless knife swirl type. I think that the swirl one does cut a bit cleaner but the straight one looks to be a lot easier to flatten and hone if it gets dinged. Time will tell I guess.
Good luck with the new grinder.
Lance