The primary reason a time period is important because a vehicle depreciates rapidly in the early years and much less as time goes by and, maintenance aside, your vehicle loses less value over long time periods, lowering the actual cost per year. You should never enter into a lease unless the form shows this cost and complies with all Federal laws. Contact your local DMV if you have any questions. Federal law doesn't require interest rates to be listed in a lease, primarily because of the complexity in the computation methods and the inability to put it in language the lessee would understand. The lease business is now so competitive it is not something you need to worry about.
RISK is involved when fixing a future value on a vehicle.
Unfortunately no one knows for sure. When you purchase, you must eventually find a buyer for your used vehicle unless you drive it till it falls apart. That will take time, money to advertise, and possible danger with unknown persons visiting you to see the vehicle and there is a potential loss by getting less than you planned. This it the biggest advantage to leasing. If future value were guaranteed, a purchase would be a sure thing because exact total cost over time would be known.