There are no hard and fast rules for this, but essentially you need to protect your property without making the guest feel that youāre being overly intrusive.
Why donāt you kill two birds with one stone, tell the guest there will be a weekly linen and towel change, this then allows you to inspect the property at the same time. You could, if you wanted, incorporate a light clean, i.e. kitchen and bathroom.
The key is communication with your guest, time and day of linen change etc. It wouldnāt do any harm to mention it in your messages pre check in, that way the guest knows what to expect. If they are resistant to this idea, then maybe you need to consider if itās going to be a good booking 
I always provide essentials to my guests such as milk, toilet paper, shower gel etc. With a long stay, this Is likely to run out. Is it my responsibility to top these items up? If so, how often should I do this?
Stuff like milk definitely no. We always put a decent supply of TP and for longer bookings make sure the shampoo and shower gel are full. I tell guests if they need any more TP, just let us know, but few do and at times we have a variety of brands in our store cupboard where theyāve just went and bought more themselves.
Should i ask the guest to sign any paper work to cover myself?
For the UK, with an Airbnb booking, I canāt think of any.
We rarely have bookings for longer than one week, our maximum is set at eleven days. We had one four week booking when we first started and decided we preferred shorter periods. Folks message us for longer stays, and occasionally we accept (Flamenco festival for example), and each time we do a linen change after seven days.
Youāll get a better idea of what is going to work once you meet your guest and have a chat about it.
JF