The real answer depends on where in Moraga you begin. The Sanders Ranch area might take you up to 25 minutes in heavy commute times. The closer in areas might be between 10 - 15 minutes. Toward the Campolindo side of town, you might chose the Lafayette BART station. The Joaquin Moraga side of town might chose the Orinda BART station.
Time will vary from there, especially during the school year. If you are leaving home at 8 to 8.30, it can be as much as 20 or 25 minutes to BART/freeeway.