Depends on the package you buy. I have a "lifetime subscription", so theoretically I'll never need to pay for an update.

...but then newer devices only work with newer versions of the service, for which you haven't got a lifetime subscription. Eventually, your old TomTom stops working and your subscription is now worthless so you have to buy a new one.

If you look at the data that the MyDrive smartphone app shows for each device, you'll notice that the "lifetime subscription" has a fixed expiry date. It appeared to be ~10 years for the devices I looked at. I could not see this information any other way except via the MyDrive app.