Tapes where I worked were kept for a certain period (30 days?) and then re-used. In the event of an incident the tapes were retained for transcript but they, too, were re-used after a certain period.
Ah! the days of ATC tapes and 8 track cassettes in cars!
It's all digital these days, at most units computers do it all and I think that there is no shelf life of a recording these days, I don't actually know what happens to them but I imagine that they're backed up and kept in a broom cupboard somewhere.