The root timer is random in a way, but by the time you have maxed you casting skils it will be either a) sticking for its stated duration b) breaking early on its own c) breaking early because of hits taken by the mob (including your casting)
Which of the three is more of a function of your level, the mobs level and if someone else is bashing away.
I have clicky pants, Ornate Leather with Savage Root, which lasts 4 minutes. I also have enhanced root as an AA. I would say most of the time, about 75% to 80%, the root lasts the full four minutes unless the mob is molested by someone. Furthermore, I can nuke mobs I have rooted and a little more than half the time the root does not break.
Lower level mobs, like four hands in ME, I can root and nuke to death without it breaking at all, but not always.