Longer than if you didn't have one.
Lifejackets have been shown to almost double survival time in colder temps, given that the floater can do just that and conserve their heat energy by not having to keep their head above water. A group can also more effectively assume a HELP (Heat Escape Lessening Position) and support injured members of the group in the middle. Doubt you'd be able to do that with a group getting tired and not wearing lifejackets.