How often should you change the oil in shock absorbers?

I'm not really into shock absorbers, I work with flowers, not cars. But from what I know, the oil in shock absorbers is usually never changed at all - they're sealed and run on the same oil for their entire lifespan. If a shock absorber leaks or makes noise, you just replace the whole thing. Maybe you should talk to a mechanic, they'll definitely give you a better answer than me.