Having read a few accident reports over the years (one C172 in the River Forth) I'd speculate that the floating time is probably measured in seconds (i.e up to a few minutes) than a longer period.
The Cessna pilot had to escape from underwater as I recall, although the high wing configuration may have been a factor in the submerging of the a/c.
One thing is for sure, the water around the British Isles is very cold at all times of year and prolonged exposure not healthy.