It varies. The most popular spots -- North Berkeley, El Cerrito Del Norte, Vallejo -- are active well before 6 AM and well after 9 AM. For the other locations, you'll have to check. There certainly none after 10 AM - the carpool lane opens to other traffic at that time. The more popular home-bound carpools on Beale St. in San Francisco (to Hercules, Vallejo, and Fairfield) are active from somewhat before 4 PM to 6 PM and later.