Hi @Juliette451,
Basically no matter what you have for a cancellation policy set for 27 nights or less, any reservation that is 28 nights or more will override that policy. So if someone books for 3 months, they would pay monthly and if they want to cancel after checking in, the following month would be non-refundable by the system but you could still provide it if you wanted to.
Here is a great Airbnb help article regarding long-term cancellations.
https://www.airbnb.com/help/article/1361
David
Superhost Ambassador ~ Host Club Community Leader ~ Experienced Co-Host