Most leases permit 12,000 to 15,000 miles a year. Drag out that old crystal ball. There are plenty of services that can tell you what the dealer paid for a new car, but what its resale value will be, after the lease has come to an end, is anybody's guess. If the residual ends up being less, the dealer eats the loss. If it ends up being even higher, you might choose to actually buy the critter at a bargain price.
Timing is everything.
Lease early in the model year to avoid mid-season price hikes and a lower residual value. In other words, it'll be cheaper. A lease is essentially a loan with a balloon payment due at the end. For the term of the lease, you'll be paying an often undisclosed amount of interest on the full purchase price of the car, less your down payment, if any. Unlike typical car and home loans, leases aren't paid off by the end of the term.