yes, you can try them anytime you want. They are the 2 main stones in my rotation right now.
On how they wear... the answer is very slowly. I've had one similar to the current ones i have that have lasted through 3 years of my serious abuse and sharpening and still have about a year left in them. These ones seem to wear a tiny bit faster, but still, for most anyone that is not sharpening professionally, i cant imagine you will run through one anytime soon.
On really coarse work, i still use my wheel (the one i have on now is about 150 grit), but then i follow the 150 grit up with this medium grit and it works great to remove the scratches quickly.