久し振り (hisashiburi) - after a long time
This will often get used in a couple of different ways based off of the main word. The first adds and "o" to the beginning of the word, which adds a level of politeness to the term. The other usage adds a casual flair, dropping the "hi", and occasionally lengthening the "s" in "sa". In either way, or in the normal expression as given, the term is used as, "Hey, it's been a while."