A really good source for determing the fastest way to get from Point A to B using public transit is http://www.511.org Take a look at their "Public Transit Trip Planner."
You can also see BART schedules and a City-based system map at http://www.bart.gov/schedules/index.aspx
Access to BART is dependent on where in those cities you live, but on average, you should give yourself around 15 minutes to got to the BART station.
Walnut Creek, Lafayette and Orinda have a BART station. From Walnut Creek to San Francisco (Embarcadero) is 34 minutes, from Lafayette is 30 minutes and from Orinda is 25 minutes. Moraga residents access BART from either Lafayette or Orinda.