Booking in advance not only ensures a better chance of getting the vehicle of choice, it can save you up to 3% on your car rental. For a below average price, Cheapflights recommends booking your car hire at least 54 days in advance.
The length of time you can book a car hire for is subject to the inventory of the supplier. When renting a vehicle from Antananarivo it should be possible to hire a car for a whole month if needed.