So you first germinate. You get a small tap root shooting down and the seed pushes up through the soil. When it emerges the seed continues to open up till the first little round leaves pop out and it starts growing the first set of ridged leaves.
This is the seedling stage, and you want cool lights really close so they don't stretch. This usually lasts 2 weeks or so, and a small breeze to strengthen the stem can help (small though). The will put out a single bladed leaf, then usually a set of 3 prongs. Soon as those 3 prongs are photosynthesizing is usually when I consider it out of seedling, but you could also take this stage a little further.
Next is veg, basically the plant is immature, and growing for size. It'll start kinda slow, and pick up as the plant develops and builds stem/leaf/root structures. In this stage you are mostly trying to get bulk out of the plant, either getting them tall, or bushing them out, depending on how you plan on growing. At any point you can push them into a flowering period, however there are other signs people look for as well to tell it's ready to flower. Some go by size, some by the presence of alternating nodes, some by the presence of pre-flowers (though I don't suggest this one).
However you decide when to flower, it will happen when the plant receives enough hours of TOTAL darkness per day. This time can vary by genetics, but a good bet 12 hours of darkness (and pretty much everyone uses this if they haven't dialed in their crops over a long period of growing). Over a period of time (sometimes as long as 2 weeks) this dark schedule will cause the plant to start developing flowers. Females will have brachts with little hairs poking out, the males get little balls on stalks that will burst after 2-4 weeks of maturing. Once flowering the females (which is what we care about) will continue to produce large brachts with pistols (hairs) as long as she does not get pollinated.
At the end of flowering the plant is preparing to die. The best way to determine the point of ripeness of the buds is to look at them under a microscope to see the ripeness of the trichomes. In the absence of this you can tell oftentimes by the ratio of hairs that have turned to orange, red, or brown. A good rule of thumb if you are unexperienced is to see when you think they are ready, then give them one more week.
You can reveg, but usually at this point in nature the plant dies.
