"long term stays" are stays of 28 nights and longer (having a special cancellation policy and payment rules, consult the HELP section voor more info) .
So set your minumum stay setting to the amount of nights you want as a minimal stay and provide a monthly discount or set fixed prices for specific months.
@Amy---Dan0 It's confusing what you are asking and I interpreted the question differently than Emiel did.
Are you asking if you can require advance notice for longer term bookings, so you don't get someone booking for a month the day before check-in, but allow no advance notice on short term bookings?
If so, no, I don't think there is any way to do that.
Not quite @Sarah977 currently we allow bookings 3 months in advance. Was wondering if I could open the calendar for 3-6 months bookings but only long term stays.