@Pete69 Well, you'll get hosts who think guests should get clean sheets every 3 days (which seems over the top to me), some who say a week, and I've even read some that say every two weeks (which I also think is a bit much, altho I often go that long on my own sheets, but I'm not a paying guest).
I never wait for guests to ask, I think it's the host's job to offer. I offer clean sheets and towel sets at the week mark. I ask them if they would like me to make up the bed, but most of my guests say they will do it themselves.
I alter the schedule to fit the length of booking, so if a guest is here for 10 days, I'll offer clean sheets at the halfway mark, 5 days, because it seems silly to offer after a week, if they only have 2 or 3 more nights left.
And I tailor it to fit individual needs as well. I had 2 guests this year who were each sick in bed for a day. I know when I've ever been sick, rolling around groggy and sweaty in bed all day, I would want fresh sheets in the morning, so I definitely offered them, regardless of which day of their stay it was, and they were gratefully accepted.
That said, lots of my guests who stay for 10 days or so have said they didn't need clean sheets at all. Now, I only host solo travelers, so it's not like they're sharing a bed with anyone or having sex romps, so their sheets stay pretty clean. But you may find all your guests wanting clean linens after a week.
Some hosts simply tell the guests that they come in once a week to vacuum and change the sheets at a time that suits the guest- this gives the host an opportunity to make sure the guests are not being total pigs in there, with food laying around attracting cockroaches or rodents, staining or damaging things and so on.