When working for a large ferry company on the Irish sea, we were taught to remain still, tucked into as tight a ball as possible with knees as close to chest as life jacket would allow and to group together in concentric circles in the water, with the most vulnerable in the middle (I would have been responsible for potentially hundreds of passengers, although if groups were in the water it really would have been a bad day!).
I can't remember the figures quoted, but my memory was around 1 hour which could be extended to many hours if people grouped together (etc) and didn't panic.
No idea what these figures are based on, but they tally with what I was told:
Survival Time in Cold Water