What your licence covers
You own every version of Neutrino you receive. The copy you downloaded is yours to keep and to run on a licensed site indefinitely — including after your update window closes, and including if we stopped selling the theme tomorrow. Nothing expires inside the theme, nothing checks a licence at runtime, and no version of Neutrino will ever stop working because a date passed.
What has a clock on it is the right to receive new versions. That is the thing you are buying a period of, and the thing that lapses. "Twelve months of updates" is routinely misread as "the theme stops working after a year" — it does not, and it cannot, because there is no mechanism in the theme that could enforce such a thing.

The practical question is therefore not will my site keep working but when will I want a newer version. In practice that is driven by Ghost: Ghost introduces breaking changes only in major releases, so a given Neutrino version keeps working until the next Ghost major arrives.