As a Survival hunter I try to use Rapid Fire at least once in a boss fight. It depends on the fight where I use it. I might wait and pop it if I think I'll need to hop into Viper Aspect, so I can regen quickly. Or if it's a fight with a lot of movement, but a few times you can plant your feet, I'll pop it when I'm stationary and getting the sniper buff.
I have a Markman friend and want to tell her how to maximize her Rapid Fire. She of course has two levels of Rapid Killing and Readiness. Yet, she doesn't use Rapid Fire on each boss. This is of course a shame. But what's the ideal situation?
It seems like she could pop it after the MT has a good threat built up, fire joyously for 15 seconds, let loose on everything else and then pop Readiness and do it all again. That takes us out to at most 60 seconds into a boss fight. But then because of two levels of Rapid Killing, the cooldown on Rapid Fire is 3 minutes. That's the same cooldown as Readiness. So at minute four she can go nuts two more times.
So four rapid fires in a five minute fight, right? Can you tell I haven't played a Marksman since they put Readiness into that tree?