we're a cosleeping family and we also wash sheets once a week (unless someone is sick, then more often...right now that sick person is me...
well, that is the quick answer. technically, i do the sheets, pillowcases and fleece 'under' blanket once a week. the mattress pad goes in once every 3-4 weeks. the duvet cover goes in every 5-6 weeks. and the comforter (hypoallergenic) goes in every 8-10 weeks.
both dh and i have allergy issues and they are definitely kept at bay with regular washing.