well I've long known that it degrades, presumably to the point of no use. this was me google:
How Long Do Audio Cassette Tapes Last?
but it reminded me of something from earlier this year about a breakthrough that can potentially store and for a long time big quantities of data:
IBM breathes life into magnetic tape with storage breakthrough
which also reminded me of the mind blowing idea that you can use DNA for storage, incredible potential amounts. But currently the read/write process is ridiculous in time factor so it remains a tech waiting for the next leap:
Harvard scientists pioneer storage of video inside DNA
I say we go back to microfiche in case there is a solar flare and all the clever machines stop working