There is no limit to how long you can keep the car unless otherwise requested by the car provider. If you wish to continue using the vehicle, simply stay subscribed and ensure that your payments are being processed, so that your account remains in good standing.
If you wish to cancel, you have to do so at least 24 hours before your subscription renews. Please return the car to the car provider at any time during your subscription that works for you both or at the renewal date/time at the latest.
You are subject to pay the minimum term set for your vehicle or a cancellation fee as explained here.