RDM Impact is one of the unique processes you have compete control over, you decide when you want it to be active and can even reset the timer for it just by using Jolt II again. Saying that, Impact has a very generous timer to begin with so there isn't really any reason to keep hold of it, just use it when it's up because there are no real downsides to it, aside from that be sure to balance your mana correctly though.