The way Portside operated in the past was to have a few shuttle buses that ran a continuous service. They would not be able to quote times. Usually people only had to wait a few minutes for the 12-seater bus to fill before it left for the ship and the next bus would be available almost immediately. With longer to travel (15 minutes, not 7), maybe they will have another bus or two.
On occasions at the end of the cruise, we wouldn't be able to get in the bus that was already loading passengers, but had to wait five minutes for the next one. The buses each had a small enclosed trailer for the main luggage.