If you miss the 6 am bus, the next departures are at 7 and 8 am.
Normally figure about 4.5 hours, but the 6 and maybe 7 am buses should do better than that because they will miss the heavy traffic. Also, the last bus to Santo Domingo may do better thanks to reduced traffic.
Keep in mind that lots of things don't run on time in the DR, but Caribe Tours does. So get to terminal with time to spare so you can get your ticket and find your bus. It can be confusing in Santo Domingo since all the announcements over loud speaker are in Spanish and it can be hard to hear your destination since it is usually one of a number of stops mentioned in same announcement. What I would suggest you do is to buy your ticket then walk back to where the buses are lined up. Normally the bus to Sosua is in parking spot 1 or 2, maybe 3, but one of the first. Stand there and wait. They will put up a sign with Sosua on it, and you can also ask any of the employees passing by which bus is for Sosua. Another advantage of waiting there is you can be one of the first on the bus so can pick your seat and decided if you want the first seat, aisle, or window. I have never taken that bus so don't know how many people do, but sometimes buses are full.
Also, Caribe Tours buses can be quite cool due to the AC, and I would expect that first bus may be one of them. so have an extra shirt or light sweater handy that you can put on if your bus is cold.
