You can veg them for as little or as long as you want.
A general rule of thumb is this :
when you put your plants into flowering they double and then triple in size.
So grow them a third of the height of your room and when you flower them there will be enough room for them.
You can flower from seed as "regrets" said above, but it's not really worth doing as you'll get little to no bud.
Hope this has helped.