It is rather complicated. 😅 As I've understood it (and I might be wrong), the setting takes into account the available RAM your machine has. Then it also checks how frequently you interact with the tab (e.g. a tab you check once a day should be hibernated a lot sooner than a tab that you check regularly). Also tabs that play audio, update frequently (e.g. mailbox), include forms, etc. are kept active for longer.
So some tabs maybe be hibernated after 10 minutes while others after a few hours.
But we do recommend keeping the Automatic option enabled. :tony_normal: