Dave pretty much summed it up. Once the ship is cleared by US Customs and Immigration, they will call over the PA system that they are ready to begin disembarking, beginning with "express departure" guests. This means that you MUST be able to carry off your own luggage without any assistance from any staff member or porter. How early you can disembark depends on when the ship arrives and when it is cleared by the authorities.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)