Hello @Ida36 ,
1. The guest can make a booking on your listing at Airbnb for the period he wants to stay at your place.
It's up to you to accept that. Long term stay also means multiple payouts.
Please read some information first:
What are some differences between long-term hosting and short-term hosting?
How do changes and cancellations work for long-term reservations?
OR
2. You can provide the guest a renting contract yourself (or via an agency). Then he will be a tennant instead of a guest, with other legal rights and facilities.
Best regards,
Emiel