ronks, I used to ride the bus to Samana regularly, but now live in Sosua. That is why I am a little fuzzy on the time. Most of the buses from Santo Domingo pass through San Francisco de Macoris. As I recall, those buses take about 4 hours to get to Sanchez, including a stop in SFdeM for refreshments.
Caribe Tours also has at least one bus to Samana which takes a different, more direct route. Don't recall the time savings, but at least 30 minutes.
BTW, from Santo Domingo you can take either Caribe Tours or Metro to Sanchez. I tend to talk about Caribe Tours because it has more buses on the route.
Over the mountain to Las Terrenas will take 30 minutes or more. Beautiful views. I say over the mountain because there is a mountain range across the middle of the Samana Peninsula. On one side of the range are Samana and Sanchez, Las Terrenas is on the other side.