The longest time that I ever had a downtime is when I was attacked and hacked and it pooped the front and backend at one time and I was down for half a day as I had to restore from the backup. I had times when the backend was down and yet the front was not.
I was a staff on a big forum that had a downtime for more than 2 days. The persistent call from users on social media handles made me to understand how important forums can be for people's lives.
I once had a downtime for over a week. I tried installing a mod and it did not go as planned and my forum started showing errors. I tried troubleshooting but it did not work and I finally had to restore the database.