I changed mine on my K9 Wee at 14000 MLS as the front sprocket was very"hooked".The rear sprocket and chain looked fine.I did it more than anything because I was off to France and didn't want any problems on the holiday.The bike had 7500 miles on it when I got it,it had been dealer serviced from new,but the state of the bike when I got it home on the trailer didn't give me much confidence in the dealer maintenance.In hindsite I wished I had only changed the front sprocket,as it has a lot more loading on it than the rear and it's a cheap way of doubling chain life.I fitted a Loobman when I renewed the chain and sprockets,what a great bit of kit for £20.The chain has only been adjusted once from new and it has now done 6000 MLS.