Generally, for a better deal at Greenville Mid-Delta Airport it’s recommended to book in a timely manner (to save up to 89%). Cheapflights recommends you book around 36 days prior to your pickup date. Booking as early as you can also increases the likelihood that you’ll get the ideal car.
The length of time you can book a rental for is subject to the inventory of the supplier. When renting a vehicle from Greenville Mid-Delta Airport it should be possible to rent a car for a whole month if needed.