Seriously although I've never filtered before how often and what sized pore filter should be used would depend upon the level of "contamination" in the initial product.
Understand also contamination could range from "heavy metals" to certain organisms such as viruses and bacteria.
Additionally it's also important to realize unless the contamination is related to a particularly virulent (potent) organism or one which is of low potency yet is in large enough quantities, infectious complication are quite uncommon.
So the filtering any AAS is used for much more than the removal of infectious junk.
Fact is, I've use labs which pay less attention to the quality of the product being delivered, which includes filtering, and it can and does make a difference.
For example I know of two labs personally which "triple filter" their products and a couple more that single filter theirs, and although I've not discovered any difference in the rates of infection, the triple filtered AAS is MUCH "smoother" or easier to PIN.
That is to say the pressure required to inject a 3X filtered product is considerably lower than one which is mono-filtered, (everything else being equal), and when small needles are being used, that difference can be significant, IME.
Jimmy