@Penelope9 I have a 2 week maximum myself, but hosts who do long-term bookings do recommend going in once a week to change out linens, add used-up amenities, and do a quick clean. This enables you to make sure the guests aren't living like pigs, keep the mess at bay, and hopfully encourages guests to keep the place from turning into a disaster area. Needs to be mentioned in the listing, also re-iterated in message to guest when they book, and a suitable time arranged for this that hopefully works for both your schedule and the guests'.
Personally, I never enter the guest space during a booking, unless they ask me to for some reason, but like I said, I have a 2 week max. Also I host in my home and the guests often leave their bedroom door open when home, as it opens onto the balcony where they have a lovely view and fresh air. So I sometimes get a glance as I walk by and can see that the guests are keeping everything tidy (plus I just tend to get guests who obviously don't like living in a mess). When I offer the linen change, I ask if they would like me to do it or do it themselves. So far they have all said they'd do it themselves, which tells me they want their privacy.
But if I did longer term bookings, I'd definitely arrange to go in and clean once every week or two..