How long to veg?

I am starting my first real grow and was growing in a closet around 5 feet high. Im growing Greenhouse alaskan ice and barneys g13 haze under 400 watts hps. considering the fact that you should keep the light 20 centimeters away from the plants, how long should i veg for?

How much do plants stretched during flowering?

I meant to say stretch

You could probably let em get 2 feet tall while vegging. They could double in height during flowering.

From one newbie to another, from what I read its one (or more) of these:

1. At least 10-12 inches...BTW: I started flowering mine at 10inches and now they are about 17 (going into 3rd week).

2. When the leaves stop growing out symmetrically. Like one leaf stem grows out above another instead of the same height.

3. After you have at least 4 sets of leaves.

Of course, my rule of thumb was whatever height it was at after a month, since its my first grow. Its weird though because all three of the above conditions were met at one month for my plants.

i would let them grow to a max of about 2ft just because like growspecialist said they can double and sometimes even triple in size during flowering.