You would need minimum 14 days if you go SF, LA and San Diego, including driving and sight seeing. We drove to SF and Berkley last last year around this time, spent three weeks total. We went down along hwy 101 and back via hwy 5. 101 is along the coast line, it was nice but nothing is nicer than here until you get into CA. Hwy 5 is faster but very boring. If you go SF only, 10-12 days might be okay.
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Note that LA area has some rain and flood issue now (we booked another flight to LA for coming Feb, so keep receiving these e-travel alerts):
http://www.cnn.com/2010/US/12/22/winter.weather/index.html 
