No one says. It's just me guessing. And I could be wrong of course.Who says that Desmond and Penny were on the same timeline as the O6? The events we saw with them could have been several weeks or even months before the O6's story begins in season 5 which would give them more than sufficient time to sail from wherever they were to England to L.A. Desmond would want to stay off the grid as much as possible. Sailing as opposed to flying would help that goal a lot more.
We don't know how long she has been in LA. We don't know exactly when he went to see Widmore. All we really know is he showed up at the same time as the O6. But the way the episodes are constructed it seem these events happened within days.