I know way back in the day, crit chance was calculated at the point at which you started casting. So that if you cast a spell with a 2 second cast time, with a crit buff that will wear off mid-cast after 1 second (mid-cast), you'd still get the benefit of the buff at the end of the cast.
I've been operating under that assumption, but recently I'm not too sure that's true, especially for Chaos Bolt. After testing on dummies with Dark Soul: Instability, I've noticed that my Chaos Bolts do not get the benefit of the buff if it wears off mid-cast.
If this is correct, I must admit I rather dislike this. I assume that stacking crit strike procs for use on a string of chaos bolts (for example popping Dark Soul when Jade Spirit and other trinkets proc while you have 3 or more burning embers) is ideal (if I'm wrong please correct me). In any event, it seems like it'd be easier and better to be able to calculate, right at the start of your cast, whether or not it will have the benefit of the buff.