[neallewis]

Depends on the surface you run on. On dirt it gets contaminated really quickly, and the o-rings get clogged up with oily gunk which reduces the efficiency of the shock as a whole, affecting performance of the car. It's really noticeable when they go "off". On astro this happens less for sure. Indoors, I find on cork tracks like Worksop they eventually get pretty dirty with black rubber particles and dust, which do contaminate the rear shocks (more than the front), but not as quickly as on dirt for sure. I try to keep my shocks clean with a brush off after every run and a thorough blast with the compressor at home after each meeting.

If I'm happy with a setup I tend to leave it for a few meetings or more in the indoor season. Outdoors I find I run on different tracks or perhaps tuning a new chassis, so tend to change oils more often.

If you strip your shocks down and the oil is really dirty, inspect them sooner next time.