I dont know where you sail but where I am you would be lucky to last 3 hrs after that it would be irrelevant how long the life jacket stayed inflated.
I sail in the Long Island Sound so according to the following resource I could have as much as several days in the warmest part of the year.
Of course now it would be only a few hours.
How does the Coast Guard decide when to call off a search-and-rescue mission? - By Christopher Beam - Slate Magazine
To determine someone's odds of survival, the Coast Guard uses software developed by the Canadian Defense Department called the Cold Exposure Survivability Model. Plug in various factors—water temperature, air temperature, the person's height and weight, garments worn, time of disappearance, access to flotation devices—and the program tells you how long the person is likely to stay alive. In general, someone floating in 50 degree water can only survive four hours (PDF). In 65 degree water, which was the temperature off the Florida coast this week, it takes the average person as many as seven hours to lose consciousness. If the water temperature is above 70 degrees, he could survive for days—that is, if he doesn't die of thirst or exposure first. (The CESM can also calculate the odds of survival if someone is lost on land in cold weather.)