The vegetative period of a cannabis plant (the bit between germination and flowering) can be as influential to a plants results as the flowering stage as in you wont yield well if the roots haven't developed properly and the key to good green growth and then healthy flowers is the root development, and that is most active during vegetative period.
In nature the cannabis plant will exhibit pre-flowers on sexually maturity and will be triggered by the amount of darkness it receives, around 11-12 hours of dark would trigger flowering but there would be variation in different strains and some would start sooner, the amount of light and dark a plant receives is the photo-period, auto-flowering strains will flower under any light regime and are triggered by age and are not included in any of this description. In a veg state photo-periodic plants would be getting 15-16 hours at most of sunshine but indoors times of 18 and 20 and more are recommended although much debate and sub-debates exist on these matters of light times, I will stick to the tried and tested.
How long to veg for is quite a common question but it is a fair one as every strain can be so different in its development, as mentioned above the amount of vegetative growth directly affects the flowering stage and some plants can grow enormous with too much veg indoors doubling or tripling in size but for roots to develop properly I wouldn't recommend less than 3-4 weeks for most strains, and remember 12/12 from seed, where a plant is forced as early as possible to flower with 12 hours of dark and light straight away still has a vegetative state where root development is more rapid as the plants also need to be sexually mature and this takes around 3-4 weeks on this light cycle but a plant grown with no real veg period like this will not develop as well and will not yield as well as one grown under 18/6 or more. The most important thing to keep in mind is head height when deciding how long, imagine the plant you have now double the size and you have a good idea of when to switch to 12/12.
A plant in vegetative state
There are many techniques to use to make a plant yield better like LST and super-cropping etc and all of them work best in the vegetative period as the plant will recover and be stronger, once the plant is triggered into flowering it will send a lot of the plants resources into the flowers even taking nutrients from itself if it is under nourished. A vegetative state can be maintained indefinitely with a cannabis plant and this is often employed to keep a mother plant to take mature cuts from to grow out under flowering conditions somewhere else.
The key to a successful grow is the start you give it and for the sake of another weeks veg you can have another half oz of weed in the end and remember you can always tie it down if it gets too big and thats covered in the LST section.
LST on a vegging plant
Plus if you keep them in smallish pots they're gonna be easier to wrap up and move out when you decide to get the camo gear on and do the deed 
