People that take ship sponsored shore excursions have less to worry about because cruise ships will usually wait on passengers that took a ship sponsored excursion. Cruise ships will usually wait for these passengers because otherwise the cruise line would have to pay for transportation and compensate the guests for leaving them.
People that are getting off the ship for non-ship sponsored excursions and for personal plans will get left by the cruise ship much sooner than a ship sponsored excursion guest. If you are traveling on personal excursions or doing your personal plans the ship will not know where you are or when you are getting back.
The cruise ship will probably wait as long as possible before deciding to leave a person, but the decision of how to long to wait, if any, is up to the captain and his officer's in the bridge. If they decide to leave the gangway out for a short or long amount of time, that is their decision. Typically, once the ship leaves the ship would not turn around for any passengers, unless the navigational bridge staff have a specific reason for turning around.
Always have an emergency contact number for your ship with you when you get off in a port of call. Many cruise lines provide a number to contact if you find yourself running late.
The easiest way to ensure that you sail away on the ship is to make sure you are on before the designated time. You are not guaranteed that the ship will wait on you and you should not purposefully be delayed to see if the ship will wait on you. Make sure you stay on a ship's time, know what time to be back onboard, and keep track of time during your day in your port of call. You are responsible for your timeliness. Enjoy your cruise and day in port!
Some Photos Of My Cruise Ship Waiting On The Masses of People That Were Late...
Here are some pictures from Carnival Triumph 2013 cruise of people still waiting to get on the cruise ship at approximately 4:30PM. This was 30 minutes after scheduled sail time and a full hour after the required be back onboard time. There were many people that were waiting in line for a long time, but there were other people that entered the Puerta Maya pier area around 4:30PM to get back onboard.