Generally, for a better deal at Castlegar Airport it’s recommended to book in a timely manner (to save up to 85%). Cheapflights recommends you book around 71 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 Castlegar Airport it should be possible to hire a car for a whole month if needed.