Currently, the cheapest month to rent a car at Miyazaki Airport is June. The most expensive time to rent a car is November. The cheapest prices for each month are shown above, but prices will vary based on availability, how far out you book your rental car, and of course, car type.
Typically, how long you can rent a car for depends on the inventory of the supplier. For Miyazaki Airport however, it is certainly possible to rent a car for an entire month.