Depends on how fast you ride and how long per day you want to be on the road. If camping I'd be more inclined to do it in 4 days: 1) San Francisco to Capitola and stay at New Brighton park, 2) Capitola to Big Sur park, 3) Big Sur to San Simeon park, and 4) San Simeon to San Luis Obispo and Amtrak back. About 80 miles/day with a shorter last day to catch the train. The route usually has the advantage of favorable winds along the coast and beautiful ocean vistas. But it's not flat, especially in the Big Sur area.