Long Term Stay Cancellation Policy

I would like to see an option for a different cancellation policy for stays of 28 days or more. If someone books a property for a month and then cancels at least 30 days prior to arrival (per our policy) they get all of their money back but the property has been tied up and lost potential bookings. I want my policy to remain as is for shorter stays but would like to see stricter options for longer stays.