Every time your user log in, schedule a backend event in 80 days to send an email. Then cancel if the user log in before.
OR
Every day, run a scheduled workflow that sends email to all users with Login Date < current date & time - 80 days
@chris.williamson1996 Your solution is great but if the app or other users can modify the user data when he’s logged out, then the value “Modified Date” will not be his last login time.