All day.
There are direct buses from Santiago to Samana. IF you do a search on this site, I am sure that the schedule I posted some time ago will come up.
This is one of pretiest DR country sighseen, I recommend you to take a break on the road to enjoy a delicious fresh fix at "Restaurant Chen" under the coconut trees, this is located at Matanzitas (Posa de bojolo, Playa gringos) Nagua.