On my previous cruise experiences we were let on after the ship was cleared of all previous passengers and the main areas cleaned and reset for embarkation. This usually was about 11-11:30 depending on how the morning went.
I have seen posts recently though that they are not embarking until after 1:00
There is usually a airport-ish waiting room with vending machines for snacks while you wait.
Sorry I couldn't be of more recent help.
Carnival Liberty - B2B - October 2014
Carnival Miracle - April 2013
Carnival Paradise - January 2012
Carnival Liberty - December 2010
Carnival Sensation - October 2009
Carnival Triumph - May 2009
Royal Caribbean - April 2008