Pretty sure invis is supposed to be random, but it seems that if it does wear off it wears off in the first few seconds, whereas if it lasts longer than a few minutes, it will last the entire time. That's my experience on p99 (I think it was more random on live, where each tick had a chance at failure).
As for buffs, that's primarily up to the lvl of the spell and, to a lesser extent, the caster.
DOTs last a set number of ticks.
I don't know about charm and harmony/lull for non-Enchanters and bards, but I think that's just random and isn't affected by charisma.
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