Resist Check and Charm Question

[Barlu]

Did some searching but couldn't find the answer so apologies if this is redundant. When you charm a mob does the game designate the duration of the charm at the time the spell lands or does it continue to check to see if it will break every tick? I know at times when I've had a mob charmed for awhile Tash will wear off. I'm wondering if that makes it more likely to break or if it just checks my charisma and the mobs MR at the time the spell lands and then assigns a duration then. Thanks in advance!