This includes time in Cumberland plus time spent getting there (driving to St. Marys more time on board the ferry). This gives visitors about six and a half hours to explore Cumberland Island. If you are willing to spend extra money and want flexibility in your schedule, you can call Lang's Marina and request a private charter to Cumberland Island.
We did it a few years ago with another couple, and it worked out perfectly. We were on the island for about 4.5 hours, but we didn't tour the houses (unless you count the ruins). But we did some nice walks, went to the beach and, of course, saw the horses. You would still have to pay the National Park fee. The people at the marina were very accommodating, especially with the schedule changes: we had to cancel our first trip for personal reasons, but without penalty or limitations as to when we could go again.