The rear disc when new is .20 inches. Service manual recommends they be replaced at anything under .18 inches. My rear disc is at .1529, after 35 months of riding and 40,000 miles. The rear has already lost 23 percent of its total thickness, which means that it's lost a lot more than that of its usable thickness (since you can't grind a disc down to nothing). There is also a lip formed on the inner surface of the disc. With that sort of wear, I'm going to replace it before it becomes a problem, because at this rate it's going to wear down to the point of warpage sooner than later.
The wear may have been accelerated when I put EBC HH pads with OEM rotors on the bike at 32,000 miles. I was leaving on a trip and didn't have time to wait for a set of OEM pads to get here, so I used the EBC ones. I probably won't use them again and stick with OEM.
I know that a lot of guys never have to change their brake discs. I've never had to change them on a bike I've owned before this. Guess there's a first time for everything.
I bought a new OEM rear disc from Rocky Mountain ATV, along with new OEM pads. They weren't the cheapest, but they've always been a very reliable company to deal with when it comes to OEM stuff. Other companies like Partzilla were cheaper, but there is a long list of customer complaints on line about them, so I opted to pay a little more and avoid any headaches.
"No matter where you go, there you are."