Lifetime map subscription is for 10 years.
@Tabb TomTom has stated that if a unit is still working and supported after 10 years the subscription will be extended upon request. The 10 year date is more or less a "placeholder".
When you purchase a navigation device or smartphone app which includes Lifetime Maps, you can download 4 or more full updates of your map every year for the lifetime of your product. Lifetime Maps are available without additional charge and for as long as the product is supported.