There are no nonstop flights between Buffalo Niagara International Airport (BUF) and San Diego International Airport (SAN). The fastest one-stop itineraries (typically via Chicago O'Hare, Detroit, Atlanta, or Dallas) take approximately 7 hours 35 minutes total travel time.
Distance: ~2,250 miles (3,620 km) great-circle.
For the most current schedules and fares, use a flight search engine (Google Flights, Kayak, airline websites) and filter by preferred connection airport.