Generally, for a better deal at Watertown Airport it’s recommended to book in a timely manner (to save up to 71%). Cheapflights recommends you book around 35 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 car hire for is subject to the inventory of the supplier. When renting a vehicle from Watertown Airport it should be possible to hire a car for a whole month if needed.