a long time; means not only that the length of the event is long but also that the event is an important one, worthy of taking up so much time.

KGT says:

This is a rather long-winded way to say "a long time" (which would be {poH nI'} in nonidiomatic speech). It implies a bit more than that, however, for it is used to mean not only that the length of an event is long but also that the event is an important one, worthy of taking up so much time. The expression comes from the well-known story of Kahless the Unforgettable, the founder of the Klingon Empire, and his brother,  Morath, who fought for the fabled "twelve days and twelve nights" because Morath had broken his word and brought shame to the family. The word used for "days,"  {pemmey}, is the plural form of {pem} ("day, daytime"), a word referring to the part of the day when it is light out (as opposed to {ram} ["night"]). Another word, {jaj} ("day") refers to the full period from dawn to dawn.