BV suppliments the regular ferries with extra vessels to handle the numbers. The regular Sorrento ferries leave on the hour, see attached website for times.
The other ferries leave when full. My 2 x experience with this was about 1/2 hr after the main vehicle ferries.
The look on the motorists faces turning up for an unavailable space on the vehicle ferries is priceless.
Watch out for your shoe cleats slipping when boarding the ferries. Also if you get on the ferry first, your bike is last to get off. So don't rush to get on the ferry first. I would also recommend getting water and using the bathroom onshore. Both water and bathrooms get a hammering on the ferries.
It pays to use your time on the ferry wisely. Rehydrate and replenish. Stretch your muscles. Eat your lunch and power bar. Rug up so you don't chill down too much. Check your tyres for embedded glass and stone chips. Mentally focus on crossing the finish line.