Typically, how long you can rent a car for depends on the inventory of the supplier. For Lake Garda however, it is certainly possible to rent a car for an entire month.
How far in advance should I rent a car in Lake Garda for the best price?
Booking in advance not only ensures a better chance of getting the vehicle of choice, it can save you up to 32% on your car rental. For a below average price, Cheapflights recommends booking your car rental at least 50 days in advance.