Not sure, but for comparison, rovers on Mars often experience failstate conditions when dust covers their solar panels over the course of a few months to a year or so. There have been instances of small twisters rolling past or over a rover and clearing off the panels.

So the deposition is probably not more than a few mm annually, if that, and maybe the material of the sample holders is dust repellant or something, but "being covered by dust" is most certainly something to consider.