Unless you found NOS Kawasaki sliders they're just aftermarket hyfax sliders...
My LTD still has the original ones with 4000 km on the odometer and I have never had a speedo cable connected on it
My drifter had just eaten through it's original white sliders when I got it 7 or 8 years ago, I put new sliders and it and have put a ton of miles on it and haven't noticed much wear from visual inspection...
If you are always riding it on decent snow, your track is too tight... If your riding a lot of gravel roads or sides of roads you could be over heating them... It's a good idea when you're riding beside a road, to take your sled over into decent snow every chance you can, sliders take a far amount of abuse and will heat up... Snow cools them and helps the track slide...
Also if the clips on your track are really badly rusted it can eat sliders...
They should last a top end or two I think, but it does depend a lot on where you ride and the machine they're on!