I die my sprocket carrier ones at high 20's (miles) but only due to them feeling notchy during a service. The wheel bearing had play in them at high 30's. You can tell by holding the wheel at 10 o'clock and 3 o'clock and wiggle it while sitting at 90 reg to bike. Any play at all and change them. Also if the wheel is off stick your finger in the hole and turn the inner race. If you feel any notchyness change them.

***UPDATE***

My local Yamaha service technician reckons that as the cush drive perishes, this will cause the two metal parts to grind together..

I told him that the rubbers hadn't really perished that much in 7,000 miles due to the ol' inner tube trick.

~When I mentioned the bearing, he said this shouldn't be going until at least 20k miles.
(This is a bike that is doing 80 miles a day in all weathers.)

I'm going to go with replacing the sprocket carrier bearing.
Also overhaul the caliper at the same time.

Thanks for all the advice, Tenners!

^That's a fair assessment.

Riding for extended periods with knackered cush rubbers, definitely shortens the life of the bearing and can cause the wear you've noticed, but there are other factors...

One thing that seriously accellerates sprocket carrier bearing wear is a misaligned chain. It only needs to be a small amount out to cause problems over 7000 miles. The problem is, you just can't rely on the scribe marks on the swingarm to get it right, they just aren't accurate enough and can only give an approximate guide.

I bought one of these several years ago...

http://i864.photobucket.com/albums/a...psf0c42b46.jpg

If I get the alignment set with the scribe marks on the swingarm, then check it again with the alignment tool, it is always out of line by a noticable margin against the alignment rod. In my case, the wheel always seems to be canted over to the right slightly using the scribe marks, but it could well be totally random and different on every bike?

I reckon having used an alignment tool, along with a Scottoiler from when the bike was new are the main reasons I got 25K out of the OE chain and sprockets and quite possibly why the carrier bearing hasn't failed yet (although I do swap the rubbers every new back tyre as well).