This is one of the most frustrating things to newbies. The first couple of weeks in dirt are the seedling stage.
You will notice very little vertical growth during this period as the plant is busy building a root system.
After a couple weeks the plant will start growing vertically and that is the start of the vegging stage.
I usually figure 4 to 5 weeks in dirt at 20/4 lights covers the seedling and vegatative stages. I also go by height.
So I figure somewhere around 32 inches is about the right size going into flowering mode. Pot size is important
too. Using smaller pots, less than 2-3 gallons, will often have plants ready to flower a week or so earlier if you are in a hurry.
Good luck, BigSteve.