How often should you change gear box oil !?

in a off-road bike .How often should you change gear box oil !?
If you ride sand dunes change oil after every ride.The silt gets in their and it is just like sand paper on the internal parts,or approx. every 500 miles.How often should you change gear box oil !?
2 Stroke or 4 stroke.


Personally I would change it once a year in a two stoke since it does not lubricate the crankshaft Once a year as oil deteriorates over time and on an off road bike it could get water contaminated ( especially if pressure washed )


or 4 stroke maybe more often if you use it loads ,
i dont know how many times ive got to say this but its ''not gearbox oil,its engine oil'...every 500 miles will do...
every 500 miles