When I'm fighting, I've been using Thundercloud past 15 seconds left but I was wondering if it was better to use Thundercloud as soon as it appears (but still past 15 seconds) or keep thundercloud until the last second to use it.

Which one ends up being the DPS increase? Wait the 9 seconds to use it to allow more ticks? Or using it right away still after 15 seconds for the big bursts?