In San Francisco, California, it's advisable to schedule AC maintenance at least once a year. Ideally, homeowners should have their AC systems inspected and serviced before the start of the cooling season, typically in the spring. Regular maintenance helps ensure optimal performance, improves energy efficiency, and extends the lifespan of the AC unit. Additionally, scheduling maintenance annually allows HVAC professionals to identify and address any potential issues before they escalate into costly repairs or system failures during the peak cooling months.