These are the UFC champions with the most total days as champions according to sportschew.com as of 7/5/2020 A.D. The link to my source is here: https://sportschew.com/longest-reigns-ufc/11/
This is by the total amount of time spent as champ of one division.
I cited my source out of integrity but I really wouldn't recommend going there as it is filled with click-bait and advertisements.