It really does depend on your recipe, the size of the batch, and your crock pot. I can't personally imagine cooking my soap for 2 hours, and I only cook it on low. I have seen people posting about hard white chunks in their soap, and I am guessing that in some of those cases the soap is overcooked. The longest I have cooked a 2 lb (weight of oils, not total) batch is about an hour, and that was a castille. Most of the time it takes about 35-45 minutes after the soap has reached thick trace. That is my experience with it, and I almost always CPHP. I do stir, and break up the solid mass in the center.
I would say use the lowest setting on your crockpot, low or keep warm, stir every so often, and keep a close eye on it. It won't take too long for you to get a "feel" for it.