Dear All,
If you had two weeks during the summer to travel around Los Angeles?
How long would you actually stay in LA? Worth going to San Francisco as well or not enough time?
Thank you
Sebastien Elbied

Do we assume you have a car?
Four nights in LA would see me out indeed. Head to San Francisco (4 nights), via Yosemite NP (3 nights + 1 travelling there) and Monterey (2 nights), would be my suggestion.
You need to pre-book all your accommodation.
Agree with #1. In my opinon, LA is somewhat overrated, a couple of days shoudl do it. Going north to Yosemite, Monterey and SF is a nice tour.
Alternatively, going south to San Diego (and Oceanside/Carlsbad) can be an option. I like San Diego a lot, Oceanside/Carlsbad are rather for a stop at the beach....

Yes, I agree pretty much with #1, LA 4 nights, Santa Barbara 1-2, Monterey 2 nights (you have to drive Highway 101 due to a mudslide on Highway 1 south of Monterey near Gorda) and SF 4 and Yosemite if you can get reservations. You might have to call daily to get a reservation for Yosemite since they book up a year in advance.

Really depends what interests you.....LA has multiple art museums, some of them worthwhile, some not. It has beach communities with surfing. LA and its inner suburbs have the most varied cuisine of anyplace other than perhaps NYC. Nice arboretum in Arcadia. Some very good hiking in the Hollywood Hills. Movie buffs can take studio tours and visit movie star cemeteries. Architecture buffs can find classic mid-century modern places, Frank Lloyd Wright houses and well preserved early 20th century places downtown as well as kitsch like Tail of the Pup (look it up) and controversial modern works like Gehry-designed concert hall. The usual outdated Hollywood stuff can be done in a few hours, at most.You can see a great deal w/o a car. Most people here don't seem to have used the Metro.
I'm not implying that LA isn't worthy of time (depending on your interests), or that it is over-rated, however out of a 14-night trip, I think four nights is a good proportion.
I'm pretty much in agreement with the above. 4-5 nights at most would be enough of LA for me. Then I'd get a car rental and head off to the Sierra Nevada region, to Sequoia/Kings canyon and Yosemite national parks. Or, I also like the Owens Valley, and driving into Yosemite via Tioga Pass.
San Francisco is great too. But for most of the summer the city gets socked in by fog, so slightly chilly. The summer weather doesn't arrive there until Sept/October.
The “Do you have a car?” question is important. Los Angeles is really the “Los Angeles area,” a sprawling metropolis of contiguous cities, of which LA is the largest. Although it is possible to get around without a car, it can be cumbersome and time consuming. You may spend more time than you want hanging around a bus stop, waiting to transfer to another bus. Many areas have poor, or no transit. Even with a car, you may find that the places you want to see are an hour’s drive (or more) apart.
I would suggest more or less what the others are saying—split the trip into three more or less equal parts:the LA area, some natural areas between LA and SF, and SF itself. You will not need a car in SF, so that willl save you some money.
If you cannot drive, then you could take a train to SF, which has a number of limitations, or just fly.