i thought it was 50 years continuous service as a Senior. If it includes cadet time I'll be applying this week.
As noted earlier, the membership does not need to be continuous. Cadet time also counts, as verified by myself with NHQ some time ago.
I joined in Feb 1964, as a cadet, and would already have my free ride, if not for a six year gap in the '80s. I have to wait until 2020 now.