It isn't needed very often at all unless some paintballs happen to break inside your hopper, which can be prevented by using fresh, good quality paint.
What i always do is take a squeegee (straight, not a battle swab) & wrap a small towel around it & just swish it around inside the hopper. Paintballs are a little greasy & over time it can build up inside hoppers & pods (and barrels).
You could also buy a pod swab (google it, i forget who makes some, maybe Blackstar) which is basically a fat squeegee.
EDIT: Tab, dang you & your Naruto ninjar skillz.