With Ford ESPs (the only kind to consider) you have up until the bumper to bumper warranty expires to purchase the ESP. After 12 months the price goes up $100.
The time and mileage are inclusive of the factory warranty. So a 7 yr, 100K mile ESP only buys you 4 yrs and 64K miles of additional bumper to bumper coverage (within the limits of the ESP covered parts) and only an additional 2 yrs, 40K miles on the powertrain warranty.