Most Motability leases run for three years although for wheelchair accessible vehicles it is usually five years. When the three-year lease runs out you can renew it with another contract.
In certain circumstances, e.g. if you have driven very few miles, or if the vehicle has had significant modifications to make it suitable for your needs, you may be able to extend your existing contract for one or two years. You need at least twelve months left on your mobility payments award to be able to take out a Motability contract.