As others have said, it can be really strain and location dependent. Some strains will give 3x stretch, so if you veg it too long and it gets too big, when you flick it over you'll have height probs. Personally if I don't know the expected stretch I veg for 30 days until they're about a foot tall, and then put them into flower.
Also, if you're topping them you might want to leave them a bit longer to recover/establish/train before flicking...