It can also depend on the port and passenger demographic, for example non-US pax can delay the whole process if they haven't been cleared prior to arriving in a US controlled port, or nationals from some countries are not permitted to go ashore in other regions.
But for the normal day to day events, Donna's answer has it all.
Next Cruise: P&O Adonia June 2014
Last Cruise: P&O Oriana Nov 2013
Golden, Grand(2), Caribbean, Sea(4+1 aborted)
Adonia(4+1), Arcadia, Artemis(2), Aurora(5), Azura, Oceana(3), Oriana(11), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(5), QE, Caronia(2), Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam