Clean towels every other day, clean sheets and pillowcases every five days or at checkout, whichever comes first.
I wash my sheets myself.
Most people don't know how to do laundry. They don't sort colors, they wash everything in hot water causing sweat stains and other stains to set, and they don't know how to pre-treat. (I pre-soak and wash my sheets in a water/vinegar/Borax/dishwashing liquid solution.)
I'm fine with them doing their own clothes however they see fit, but I like my sheets to look brand new.
I also iron the border of the top sheet and pillow cases so that I don't have that weird crease/fold that every top sheet and pillow case seems to get.
I turn down beds and clean bathrooms upon request but push hard with extended stays to let me in to change the sheets and scrub the bathroom. I find that if I can clean the bath at least every five days, I have no significant build up of scum in the shower. It also allows me to do a quick inventory to see if my "no food in the room" rules are being observed.
I don't accept stays of more than 15 days.
I offer a private bedroom and bath in a shared house so it's very easy for me to be able to do this.
If I were renting off site, I'd do what other suggested and leave x number of sets of sheets and towels on the premises and would provide a separate hamper where they can leave the sheets for me to do once they check out.