There are two, both across the Bomba:
1. 7:30 am, be there 7:15
2. 1:30 pm, be there 1:15
The second goes directly to Las Terranas, with the first (earlier) you must change to a guagua/publico near Sanchez (they a waiting for the bus there).
I'd prefer the early one, the later ist offen very full and hot and your luggage will be on the roof (but safely).
Both take about 4 hours, making a 10 min stop in Nagua.
Be careful to take the right bus, sometimes the guagua's to Rio San Juan try to take you saying they go to LT.