Safe to stay on Catalina - for how long?

Can anyone advise me as to for how long it would usually be safe to hold off upgrading?
For several reasons (just 128 GB disk space, FineReader OCR problems) I want to wait with the upgrade to Big Sur untill I get that new desktop Mac, and then keep the old Macbook Air for travelling.
But for how long will it be safe to keep that old system on the net, and would you expect any trouble accessing the same iCloud data from both machines on two separate versions af the MacOS?
I seem to remember that Apple usually supports security updates on older systems but for how long?
Thanks in advance for usefull suggestions!

Security Updates are only provided to the current system and two previous, but there is evidence that Apple doesn’t always patch all vulnerabilities on those two previous.

One other consideration is that Big Sur is generally much more finished and stable today than Catalina will ever be. That’s not to say that there aren’t some hurdles with Big Sur, but most users found it to be a smoother update than many previous ones have been. You’ll find a recent discussion of Big Sur issues in [Small] Annoyances in Big Sur.

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Thanks, Al, I should have said a long time ago. So my conclusion is I’ll leave the Macbook Air on Catalina for now and get myself a Mac Mini which will also solve my storage problems.