With the advent of custom User Interfaces you can now make custom in-game timers for your mez and slow spells. Almost all high end mezzes have a base of 54 seconds or so. I found a site where I could make a digital timer tailored to what time I wanted to set it for. The bar simply counts down on a 54 second timer, so I don't have to remember where I cast the spell on a clock.
In the old days I got razzed by my friends for having a three in one kitchen baking timer at my desk. That got laborious, but it taught me the time after about a year of what a normal mezz should stick on a mob.
In pinch situations or when you need to keep a mob mezz LOCKED, you need a timer to tell you when to recast a mezz (before) it breaks.
My timer is in place of my /gems game so I can just pull it up when I want.
GL all