Longer veg will give you bigger final yield, but as Switcher said many run 12/12 from seed.
Most people running SOG only veg 2 weeks. For SCROG I usually veg at least 4 weeks though.
You can do lots of plants with short veg or few plants with long veg and get same harvest. Whichever works best for you.
Pictured is my current run of Granddaddy Purp at 53 days.
These plants had zero veg. The cuts were taken and the dome placed directly into 12/12. This was done to sex the seed mothers as quickly as possible. I'm guessing they will yield 1/2 zip per plant.