there are two arguaments here.
lets draw a parallel to underwear.
HOW OFTEN SHOULD WE CHANGE UNDERWEAR?
well, some might say, wait untill its dirty, then change. some might say efore it gets to that stage.
well in this case, its up to you.
if you change everyday, there really isnt anyharm
if you change weekly, there is also no harm, but if you change after a month lets say and the water juts isnt flowin anymore cos its so clogged and dirty.... then thats no good.
as note, polyester filterwool, all you have to do is pray it with a hose, you can re use them, in fact, i reuse all my filter wool (im a cheap ass asian) becaues i find no difference or advantage to not, i clean it, re use it, and it catches dirt like it was new.
i have purchased only 1 big bag if filterwooll a year, only when its really bad then i put new wool in there.
but for a internal. generally you should have sponges not wool. if you have wool then its guranteedto clog asap. internals idont think are designed for filterwool but only course sponges anyways.