@Rachel1845 All you have to do is increase your maximum stay length in your settings. And also your minimum if you don't want any short term bookings.
But you may want to rethink offering long term stays. Airbnb only guarantees the first month's payment. If the guest reneges on future payments, Airbnb will wash their hands of it, you can be stuck with a squatter who has then stayed long enough to fall under landlord/tenant laws and you could end up with a lengthy eviction process.
There are savvy guests who know this, and will play it.
That, plus the absence of a real security deposit and Airbnb's lack of vetting procedures for guests, makes long term booking through Airbnb a risky proposition.
If you want to rent long term, there are better ways to do that than through a platform that provides no security for these types of bookings.