Leases come with a pre-set mileage allowance per year. A three year lease with a 12,000/year mileage allowance should be returned with less than 36,000 miles to avoid overage charges.
Under-mileage: If your estimated mileage will be under your allowance, you can just return the vehicle at the end of the lease, however there is no credit for being under the mileage in the lease contract. If you purchased additional mileage (but didn’t use it), this is sometimes refundable.
Over-mileage: If your estimated mileage will be over your allowance you have three options—either arrange to drive the vehicle less, pay the mileage penalty at the end of the lease (typically $0.15-$0.30 per mile depending on the brand) or buy the vehicle at the end of the lease.