October is a nice month. It's like Indian summer there. As to how long, that's hard to say because it depends on what you want to do/see and how long you want to spend at each spot. Driving along the PCH (Pacific Coast Highway) takes longer than the other routes, especially Big Sur area. It's slow go. Need to be aware of rock slides in Big Sur because that certainly closes it down. Also, wild fires. Big Sur had one not too long ago and there was no driving through. I lived on Monterey Peninsula for 16 years. I know the area well (Carmel, Pebble Beach, Carmel by the Sea, Carmel Highlands, Big Sur), if you want more info regarding specifics I'd be happy to share. Another nice stop would be Santa Barbara and Solvang. Solvang is cute. Have a great trip. P.S. Several years ago friends of ours did the coast drive from LA to SF, they spent a minimum of 10 days.