There is a sign in Puerto Plata that says 45km to Santiago.
From Cofresi exit just 4km west of Puerto Plata to the first roundabout at Ochoa Motors just outside Santiago, it takes about 40 to 50 minutes depending on the horsepower of your car, your driving skills (passing), and how many slow trucks you get stuck behind on the mountain part of the road.
It can take another 15-25 minutes to get to the other side of Santiago with traffic.