How do you feel about driving? 8 hours max or as little as 6 depending on which route you take, what time you go and where in LA you start. That's not allowing for sightseeing. 12 hours would give you a great day.
Fastest route is Hiway 5 (also the most boring). Lovely route (remember this is all just my opinion) Hiway 101 down the central coast--through history and wine. Longest route, maybe more than 12 hours, Hiway 1, the most scenic (read: beautiful). As far as I'm concerned, we never fly anymore if there are other options. What a hassle! (And I fly in my profession. Hate it!) almost 8 years ago
Where are you going to be? Where are you staying? How long will you be there? You won't want to park downtown if you're going to be i the neighborhoods and vice versa. So it's really important to know an itinerary.
Are you staying in a hotel with parking? Public transportation is fantastic in the City. Leave your car and travel like a native. Are you staying outside the city and going in and out every day? If so, it depends on what area you're in and where you want to go.
More information is needed before answering your question in a way that will help you. over 7 years ago