You do it when it's dirty.. but some dust will not affect performance so much that cleaning is REALLY needed..
I cleaned mine fist time after 10 years, then again after 5 and then after accident when secondary get smashed and pieces of glass fell on the primary.
Now I have some marks on it, but mirror still performs OK and I have big plans for it when I retire
(it is 10" f5.6 Coulter Optical, I bought it back in 1980)