I understand this is a difficult (if not impossible) question to answer. Please feel free to make an educated guess!
If a theme is no longer recieving updates, but you continue updating WordPress, how long would you say until something theme-related would probably stop working? Three updates? Every whole number WP version (i.e. version 5.0, 6.0)?
After how many WP updates should you begin to worry?
Same question but in a different way: if you stop updating your theme and WP, how fast will it become vulnerable to hackers etc?
It completely depends on the theme itself and what updates WP makes.
We still use a theme for a long long time ago and have never had any issue with updating WordPress but in some cases a file released last week could have issues with an update from WP the next.
Bear in mind there are several other factors which impact this e.g. having security plugins, how reliable/secure is your hosting and so on.
Even if the file is removed - if the author is still active then there’s no harm emailing them and asking the question of what owuld happen if you update.