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?