Up to 500 years according to a Yahoo article. We need to kill these beasts off and soon. They're probably the reason the Viking colony in Greenland collapsed and the Thule Culture disappeared about the same time. The poor souls were eaten by the parents and grandparents of these foul beasts. No doubt the methane from their ancient gaseous guts are the real reason the Greenland Ice Sheet is melting.
Man the boats, Mr. Fansome and bring along your Japanese mini-sub. We may have to go under the ice to blast the things. The things we do for humanity.