I always change the sheets. WHen a guest leaves, I put fresh sheets on and the bed is covered with a comforter that's pretty but too stiff to sleep with. When a new guest comes, I pull off the stiff comforter and add soft blankets. If it's been a long time, though, I do wash the sheets again.
I usually do our sheets once a week, sometimes 10 days if I'm really lazy
When we were battling fleas, all the bedding got washed about 8 times in two weeks
