I also found the following that may help to answer this question. "If you purchase a nüMaps Lifetime subscription (sold separately or bundled together with certain GPS models), you will receive up to 4 map data updates per year, when and as such updates are made available on Garmin's website, for 1 compatible Garmin product until your product's useful life expires or Garmin no longer receives map data from its third party supplier, whichever is shorter. A product's "useful life" means the period during which the product (a) has sufficient memory capacity and other required technical capabilities to utilize current map data and (b) is capable of operating as intended without major repairs. A product will be deemed to be out of service and its useful life to be ended if no updates have been downloaded for such product for a period of 24 months or more. Unless otherwise stated, the updates you receive under the subscription will be updates to the same geographic map data originally included with your Garmin product when originally purchased. In some instances, your Garmin product might not have sufficient memory remaining for you to load an update to the map data, in which case you will need to either (a) select reduced map data coverage for your updates, or (b) purchase separately a microSD™/SD™ card (if and as applicable to your Garmin product) and load all or a portion of the map data coverage for your updates to the card and insert the card into the microSD/SD card slot contained in your Garmin product. If neither of the measures in (a) or (b) can be used to address your product's lack of sufficient remaining memory, then Garmin may conclude that the "useful life" of your product has expired. Garmin may terminate your nüMaps Lifetime subscription at any time if you violate any of the terms of this agreement or your subscription. Your nüMaps Lifetime subscription may not be transferred to another Garmin product". Hope this helps!
According to Garmin "A nuMaps lifetime update is a subscription to every future nuMaps onetime update for the useable life of the product. Each nuMaps onetime update has an expiry date because it only stays on the Garmin servers for 12 months. It is not necessary to have a specific nuMaps onetime update accessible for more than 12 months, as there will be newer updates available at that point in time. The nuMaps lifetime subscription itself does not have an expiry date." Looking through their website, they don't actually state how long the useful life of a product line is. I happen to have one Garmin that is 7yrs old and still is getting map updates via the lifetime subscription I purchased back then. I know that doesn't actually answer your question, but hopefully it helps a bit.