There's a little checkbox in your Settings now that says, "Email me warnings of impending apocalypses." I thought this was pretty self-explanatory, but there have been questions, so I guess not. What it does is send you an email seven days before your nation will cease to exist due to inactivity. Which happens if you don't log in for 28 days, or 60 days with Vacation mode turned on.
(Deleted nations can be restored again. They just don't exist within the gameworld in the meantime.)
So basically: the checkbox means you'll get emailed if you don't log in for 21 days. Or 53 days in Vacation mode.
That actually wasn't self-explanatory at all, was it? I see that now.