Without any reduction Trueshot is a 15 second buff with a 3 minute cooldown. That's an up-time of 8.3% if you used it on CD every-time.
There are 3 ways to reduce the CD of Trueshot:
- Quick Shot, Artifact Trait x3: -30sec CD (x3 +relics with this trait)
- Legendary Boots: roughly -30sec CD
- T19 2piece Bonus: Every 20 Focus you spend reduces the cooldown of Trueshot by 1 sec [not yet in-game]
If you have 4 points in Quick Shot (3 levels +1 relic) then your Trueshot CD is going to be down by 40secs to about 2min20sec. This is an up-time of 10.7%.
If you have the Boots and 4 points in Quick Shot (3 levels +1 relic) then your Trueshot CD is going to be roughly halved, down to about 1min50sec. This is an up-time of 15%.
Chuck in the T19 2piece bonus on top of this (I'm not sure how much cooldown reduction they will provide on average, perhaps another 30sec?) are we going to be getting to the point where we will have a ~1min10sec CD on Trueshot. At this point would it be viable to priotise Quick Shot traits on relics to bring the cooldown down to ~50sec? That's a potential up-time of nearly 30%.
In this scenario you could use Trueshot on every Mythic+ pack and benefit from execute crit bonus and your DPS would skyrocket. You could also align it with Murder of Crows, ensuring you have a Trueshot up for every time you cast this (long fight ST). Plenty more applications I'm sure.
tl;dr - potential with legendary, T19 + QS relics to have <1min CD on Trueshot. Too good to be true?!