20-22,000 very easy km.s for me as well.
In hind sight, that was probably too long.
To check simply slacken the chain. Grip the sprocket and attempt to twist, radially.
I had about 1/4 " of movement when I noticed it, and replaced the rubbers immediately.
I replaced mine with the OEM rubbers because the urethane aftermarket ones have a tendancy to transfer vibrations from the wheel through the rest of the bike.
Don't forget , they're there to absorb torque. (The Weak Link, So To Speak).
If that cushion is replaced with a less absorbant one, the energy generated is transferred to the chain, wheel bearings, output shaft bearings, transmission, etc, possibly contributing to pre-mature wear.
IMO, unless your trying to cut quarter mile times, or get a better drive out of the final turn before the finish line,...
...the stock rubbers are the way for a street ridden bike to go.