There is no HV cool down, you can Not predict when he will cast it. from what I notice ADS HV interval between 15 sec to 45 sec, he can cast two aoe skills in between or no other skill at all. You cannot maintain your gl3 if you are on silence duty, which will cost 50% DPS lose.
Wait 5 sec in rotation will cause another 30-50% DPS lose. Still you are not guaranteed to get silence succeed.