According to most instructional grow FAQs, DVDs etc, the components needed in a good grow are genetics, light, air/co2, humidity, water/nutrient and growing medium. They often speak of 3 week vegging periods for clones or 6 week vegging for seedlings. These are but arbitrary numbers and if adhered to can lead to lots of problems once you switch to flowering.
The best thing you can do for your plants to give them all the tools to grow successful buds is to allow that root area to form properly. As is stated in the problems sections of most grow sites, overwatering is the most common factor with new growers. In fact its a common mistake even with experienced growers. There are plenty of instructions on how to get your plants to recover from overwatering/nutrient, the trick to it is that if you do see the affects of over watering or nutrient burn, apart from 'fixing' the problems, that you give the plants time to recover before switching them to bud.
Sick plants need to fully recover before switching them, especially plants that have had pest infestations. These may need another 3 weeks from the time the pests were erradicated in order to fully recover from the attacks.
Without a happy root system, plants will suffer once you put them into flowering. Roots need to fill most of the grow medium before switching. Signs of problems in the root area are slow growth, burnt leaf tips, leaves that are washed out in patches, plants that emit very strong odor during vegging, and pots that remain heavy for a long period of time showing that the plants are not using much from the grow medium.
What you need is for the roots to occupy a large area of the space in the pots, a sign this is happening is that you see the lower branches begin to spread out and the tips extend past the edge of your pots. Eventually this occurs all the way up the plant. Fan leaves tend to extend out as far as the roots are extending out in the pots. This is a good sign that your plants are almost ready to switch.
Next you will see them begin to uptake nutrient faster, your watering routine will increase, and increased watering does not lead to leaves burning. This is also a good sign that your plants are happily preparing to be switched and that the root area is hardened up enough to nutrients to be able to handle the budding period.
For clones this can take between 3 to 6 weeks or longer once they are potted in their adult pots.
For seedlings its a little different. Unlike clones which are cuttings from an original seedling (and can have an adult age of years old from the original seedling), seedlings take a lot longer to get ready to switch to flowwering. Some grow videos give it 6-8 weeks in vegging, in fact it can be up to 12 to 16 weeks for some seedlings to form all of their adult parts ready to handle budding. Some seedlings grown to make mother plants have taken up to 6 months to mature enough to handle heavy cloning.
Although I have mentioned some time periods, your eyes need to eventually be the guide to tell you when they are ready or not.
