Longest that I'm aware of was Kamikazi (may be two i's or two z's, don't recall spelling and would have to check my friends list in game to confirm) on Chillrend who held for about 7 months as it was DC's buff campaign (back when PvP buffs worked outside of Cyrodiil and all the trials progression groups demanded 100% one-colour campaigns. Her dethrone was my first time as emp long, long, long ago.

If you see this Kami, grats on the baby!

Shortest was AOE BBQ for his first time as emp when I was running 6 full groups of 24 in TS, and he lasted 7 minutes.

Most of my own times as emp in the 2016-2018 era (most in Trueflame, and was crowned almost every day sometimes multiple times per day for about 2 years) were about 12-15 hours, being crowned by the overnight Oceanic Venatus crew, then we held until mid way through prime time being double-teamed (as we should have been).

The most challenging to hold was for @Lightingale who was the last empress of Haderus. Back then if you held emperor / empress when a campaign was closed, you got to keep the title. We held for about 40 hours until the servers were shut down, and she kept the title for a few years until they removed them from all non-active emperors. Apparently there's a new way to do that - if you get emperor on one toon, swap to all other toons and set the title, then you won't lose the title on those other toons until you manually change it, even after being dethroned on the toon that achieved emperor when their title changes to Former Emperor. I'm not sure if that's a feature or a bug, but if it's a bug @ZOS_Kevin please pass that along.