It will vary slighly from one airline to another. It will also depend upon whether you fly into La Romana, Puerto Plata or Punta Cana. Flight durations also vary depending upon the direction in which you're flying. As a guide, however, here are some flight times from Monarch's website:
Depart Manchester 1000. Arrive Puerto Plata 1350.
Flight time = 7hours 50 minutes.
Depart Puerto Plata 1535. Arrive Manchester 0455.
Flight time = 9 hours 20 minutes