Based on my own experience and the posts in this thread, it appears that the answer varies from ship to ship, and even then, from itinerary to itinerary. And even then, it can change.
What DH and I tend to do is to walk over to Red Frog Pub relatively soon after boarding and check it out. If we don't see a happy hour listed on the sandwich board out in front of the pub, we walk in and ask. They might not be open for service yet, but they will know the answer to my question.