It’s a long time, 37 seconds.
Rose is persuaded by Jack to get off the ledge in one of the film’s earlier moments – just as she slips and has just Jack grab onto her hand while dangling above the sea. Holding someone up for a whole 37 seconds isn’t simple, thus Rose’s chances were slim.
She wasn’t likely doing any arduous labour because she was a wealthy woman in the 1910s, her outfit would have made it more difficult, and she was dangling – which would have made holding her for that long nearly impossible. But what would the film be about if she died at that point?