Hello @Christian4328
I'm afraid I'm not a bearer of good news so please don't get mad! But anyone booking a long term rental on Airbnb has to pay up for the first 30 days and can then shorten their stay (no permission needed by the host) if they give the host 30 days notice. See link below of how easily guests can shorten their stays:
https://www.airbnb.com/help/article/475
So there is never a guarantee that the guest will stay as long as planned I'm afraid and no compensation will be coming your way as Airbnb will tell you that you agreed to their terms when you accepted this reservation. Your calendar will be freed up obviously so I really hope you receive bookings to compensate in some way.
Here is an excellent guide on long term hosting on Airbnb and some of the pitfalls to avoid:
https://community.withairbnb.com/t5/Community-Guides/Monthly-Stays-on-Airbnb-Guide-Revised/td-p/2031...
All the best
Joëlle