I take it you'll be either in Long Beach or the Newport Beach area? (There are ferries from both).
Living in Monterey county, I always figure 6 hours to downtown LA. But to the areas mentioned above, you have to add 1/2 hour or an hour, so 7 hours is about right if you take I5 and not the coastal route.
The best route is 5N to 46W to 101N to 68W to Monterey. That should take you about 7 hours. You can also take 5N all the way to 152W over Pacheco pass, but then you have to take 101/156/1 back south. Still, it's about the same time because you take I5 further, which is a faster road.
Taking coastal Highway 1 all the way is beautiful beyond description, but will take you much longer. How long? Depends how many times you get out of your car to ooh and aah over the scenery. But I'd leave myself 9 or 10 hours on that route, to stop and enjoy it on the way.
HTH,
Bob