With the debuff time, we have no chance to keep the debuff up and use all our builder with cd/legendary buffs/debuffs.
The perfect time would be 20sec, so it matches the time from Setting Moon. If that would be, for whatever reason, too strong, up it to 9 or better 12sec (with 12sec it would be no "big" problem to use Tranquility/Death Touch on CD and use a buff/debuff Legendary like Shock Pulse or Seasond Strike)so you don't risk loosing the 20% from Precision Strikes.

p.s. calling the debuff Precision Strikes might not have been the best idea, since it is also the name of the level 64 Mastery