Santa Cruz - roughly an hour on the crappiest freeway in the world. i hate driving 17.
Carmel/Monterey - nearly 2 hours. water is FREEZING. but nice place to go and lots of other stuff to do.
SF beaches - about an hour. all their beaches suck. no sand, cold water.
My friends and I usually go up to Half Moon Bay. Its prettier, cleaner, and many different areas of the beach to go on. We're usually in Montara but sometimes we'll stop a little earlier if its nicer and less busy. It takes maybe an hour to get there too, because of the road you have to take once you're off 280/101.