It varies with the colleges. Usually from late February through early April. The busiest time seems to be mid March from my observations for cruises. I cruised during mid March on the Pride and it was very entertaining. That was 6 years ago though. They were not too bad, a few got a little obnoxious. With Carnival's drinking age limit, and booking age restrictions it is not as bad as it use to be.
There were a lot of buckets of beer though!