It is worth it to use a Thundercloud proc after 1 tick. So basically you can cast it whenever you get it, as long as it doesn't somehow mess you up in a rotation for whatever reason.
edit: Sorry misunderstood the question. Technically, we don't really know an exact time for it to be ideal to clip the dot and reset it. I personally just refresh it if I has less than half the time left, if I haven't gotten a proc at that point, I want to refresh my chances.
