For me it varies how long I veg for, typically around 6 to 8 weeks, but mostly dependant on plant size for that decision. I let em get fairly big before flip, but not giant.
In general I don't flip til they've sexed either. After sexing I'll up pot them into their final pot, let em fill it in a week or so then flip. Usually having been topped a time or three by then.
About 2 ish ft tall is when I send em to flower. Typically gives me 6 to 8 oz per plant, and stays under 5 ft height.