If you take Mint LTS as an example, then it's repositories are current, until the next LTS is released.
that is a time frame of 3-4 years or so, then the cycle starts again, if you continued with that older LTS, then no further updates would be made available for you to download. Your O/S would still continue to be quite usable for a few years after that.
Likewise, Mint 8 would be updated from the repositories, for about a year or so, but not as long as the LTS support is.
after that, it would still be quite usable for a few years after that.
for example, you can still use Firefox2.0, today, if you had no way of updating it,
but it may not display some sites correctly, anymore.
However, you could visit her, say twice in five years, to update it manually