Most car hire companies have a rental period of 24h, which means you will be charged for each 24h period you have the car, even if you don't use the car for a full 24h. To save money, try and make your booking so that you return the car as close to the pick-up time as possible. Generally, the cars all have a full-to-full policy, which means, you will get the car with a full tank of petrol, and you need to return it with a full tank. Make sure of this or any extra charges there may be when you book through KAYAK. Search rental cars on KAYAK