Booking early almost always ensures you have more options of car models to choose from, as well as prices reduced by up to 30% to make it a great deal. Try to book at least 90 days prior to when you need the car rental to close on a rate below the average.
The length of time you can book a rental for is subject to the inventory of the supplier. When renting a vehicle from Piloto Civil Norberto Fernandez International it should be possible to rent a car for a whole month if needed.