Ideally, you should rebuild them every season, but very few people do. I have heard every 1000 mi. for Fox Shocks depending on how hard you beat on them. Otherwise, by the 2000 mile mark i would recommend it. It makes a huge difference when you really start jumping and landing hard. I get mine done on my 440 if i feel them start to fade. Bad seals will leak. Dirt can also pit the shaft and allow moisture inside, and the Nitrogen charge can escape creating a crappy ride. Polaris dealer charges me $20 per shock, unless some of the seals are worn or cracked. Clicker-type shocks and external reservior can cost more to have done.