Time in port varies by itinerary. In most ports the ship is docked somewhere from 7 to 10, maybe 12, hours, especially in the Caribbean. The sponsored excursions should get you back to the ship on time - barring any serious unforeseen incidents. Be mindful of the time if you are out on your own.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013