@Audrey39
Super late response, but just saw this thread.
I host long term guests, so anywhere from a few weeks to a few months.
I give my guests two options:
1. I can clean their rooms and change and launder their linens and towels for a small fee. How often I would do this would depend on the length of their stay and how often they want me to do it. For a stay of three weeks, I would suggest to them half way through, i.e. after 1.5 weeks.
2. They can do this themselves, in which case I make sure they have an extra set of everything, BUT make it clear that, if they choose this option, they just actually wash the stuff (my long term guests have access to the laundry facilities). Most who go for this option do it.
However, I recently had a guest who said she would and then used all the linens and all the towels and never washed any of it. She checked out late, even though she knew I had another guest checking in, so all that left me in a bit of a rush. Still, I always have at least three sets for every room, so I wasn't caught out. That reminded me to remind the guests!
Some guests have just given me their linens when they changed the bed and I have washed them at my convenience as they have an extra set to use in the meantime. That is a good solution if you don't want your guests using the washing machine.
As for the initial question in the thread, yes, I wash the duvet cover in between every guest. Perhaps not all hotels do this, but I don't care. To me it's just basic. Even if one guest thinks it's unnecessary, you can bet most won't agree.