I think it is about eleven or twelve hours. Mrs rampart and I fly London - New York and London - Dallas, and they range about ten or eleven hours. It would be a bit longer to go to Los Angeles, I imagine.
Its about eleven hours on the way there and ten on the way back .... I take this route four or five times a year and it goes quite quickly. I find if I just re-set my watch to L A time and pretend nothing has happened I don't get jet lag at all, although on the way back I sometimes feel a bit spaced out for a few days.