It depends on where you are coming from and what mode of transportation. I did a trip there from Cabarete. A tour bus picked us up and then stopped at every All-Inclusive around Puerto Plata. On the way back we stopped at a tourist trap to buy rum and cigars. It was about 4 hours each way. At the first AI, on the way back, we got off at the first AI and took a taxi back to Cabarete.
The rafting itself was about 2 hours. It was fun, but no way is it worth 8 hours on a bus (IMHO). If you are closer or can rent a car, it would be a nice trip. The mountains are beautiful but know that driving is a challenge due to blind spots and people not following traffic rules.