I've had same dark blue covers [from West Marine] on my fenders for 10 years. Still in pretty darn good condition.
When at berth I spray them with nozzle on a pressure hose while they hang off side of boat. The pressure water gets the covered fended twisting very quickly on its line. So... as it winds its line up tightly in one direction and eventually slows to a stop... I simply reverse the side of fender to which I apply the pressure water stream. Fender gets twisting back and forth so fast that centrifugal force throws off the water along with dirt and other contaminants [like spider webs]... sort of like the spin cycle in a washing machine!
Before having covers and maintaining fender covers by cleaning them as mentioned above... I used to spend quite a bit of time scrubbing clean my fenders at least once a year. Since putting on the covers and pressure water spraying them off - not once in the last ten years have I needed to scrub a fender clean.
One word of caution about cleaning fender covers like I do: Use only standard hose nozzle water pressure and not a pressure washer. I learned real fast that pressure washer will tear apart the fender cover's fabric.
Also, I noted in this thread where someone mentioned that fender cover affects the finish on their hull. I find no abrasion at all on our boat's gel coat. Matter of fact... for years before using cloth fabric fender covers I would too often need to buff out marks on hull from a dirty rubber fender itself.
10 years of reduced efforts and consistent OK looks for fenders has made me a believer in good quality cloth fender covers!