"Rather poor" is putting it mildly. This sent me down a sort rabbit hole. From a Stack Exchange discussion[0] it was a short trip to exceedingly technical discussion about using QAM encoding[1] to really beef-up the storage capability.
With the wacky QAM encoding tt looks like maybe 20MB per C90 cassette (and 90 minutes to "read" it back).
It's interesting. I wouldn't dare to go beyond a 1500 baud signaling rate, but, then, audio tape is amenable to QAM, and that could multiply the transmission to 16 bits per token or more depending on the quality of the tape and recorder.
I would be careful with that, however. If you are archiving your data, it's because you like it and, if you like your data, you want it to be readable a long time from now. I'd suggest vinyl records rather than tapes, as they are very robust, and can be read without physical contact.
I have a hundred or so such tapes that contain Commodore PET programs from the early 1980s. Last time I tried to read them (about 10 years ago, when they were about 35 years old), I had...mixed results.
Part of that may be the tape drive (about 40 years old) but the reality is that consumer level cassette tapes aren't built to last: magnetic fields weaken, coating flakes off, tape stretches, and other factors prevent these from being storage solutions beyond 10-20 years (my guess), if that.
They might be a fun nostalgic diversion for listening to old music, where the audio degradation is part of the experience, but for data, they're a non-starter in my book.
Somewhat related, I think there were some projects to use VHS video cassettes for data storage too. It was much better than C cassettes, but still a very far cry from what one would consider worthwhile these days. IIRC a couple of GB per cassette?
LTO tape tech has gotten into pretty nutty territory - in order to achieve its density and speed. It wasn't "easy". So, so far away from C90 technology.
For example, using something like ggwave [1]. I guess that would last way more than 5 years (although the data density is rather poor).
[1] https://github.com/ggerganov/ggwave