@Krischan0
The are entitled to whatever your long term stay cancellation policy was when they booked. Airbnb may try to get you to refund them after they cancel, but you are NOT required to.
There is a common scam with monthly bookings. Guests will try to get the Host to give them a bigger discount based on a longer stay...say 3 months. They only intend to stay for 1 month, but want the bigger discount for 3 month stay. You tie up your calendar for 3 months for them (sometimes months in advance). However, after one month they want to cancel or shorten the stay and expect a refund for the unused nights. Meanwhile, you weren't able to get bookings for the 3 months they had originally booked. If you don't refund them, some will start making up stuff (not clean, safety issue, bad smells, mold, you discriminated against them...the list goes on). You only get paid by the month for these stays; not paid up front for the entire stay.
Here is a Host Guide for Monthly Stays (scroll down to scams):
Understanding Monthly Stays on Airbnb
https://community.withairbnb.com/t5/Community-Guides/Monthly-Stays-on-Airbnb-Guide-Revised/td-p/2031...