Have flown in the LA basin quite a bit and the marine layer is usually gone by midday, although it has been known to hang about longer. If you doing 100PIC then why not add an FAA IR rather than just burn holes?
Another thing to note is that it conversly fogs in in the evening due to the basin so be careful as you may not be able to get back in unless you have an IR and end up diverting elsewhere to spend the night.
J.