- Sorry, I can't give you a number in seconds.
But I have to say a few things.
First, it is a bad idea to have automatic updates and upgrades.
Like Old Toad said, sometimes updates cause issues. It is much safer to do updates in your schedule.
An OS upgrade is a new OS - like from Catalina (10.15) to Big Sur (11).
An OS update is, for example, going from 11.1 to 11.2 or 11.6 - new iterations of the same OS.
It is a very bad idea to let an upgrade happen automatically - especially if you don't have an active backup strategy.
See it this way: next month you might wake up and your Big Sur mac could suddenly be runnning Monterey.
What if you rely on an application that does not have a compatible version? Better to do due diligence and upgrade after you are confident that your needs will be met by the new OS.