The only answer is time. But the minimum will depend on the pops generating turmoil.
If it’s a state you conquered you just have to wait, but pops can also generate turmoil from political actions and also being discriminated.
Also violent suppression does not reduce turmoil. It only reduces the effect of turmoil. Though something can be said by +2% mortality per level of turmoil. So I guess in a way you’re killing off a small portion of the turmoil pops. No idea if this pisses them off more and slows things down or actually speeds up getting rid of dissenters.