I came across this blog and thought of all of you guys who ask this... Confusing question... Enjoy
Cannabis Light Schedules: When should I change my light schedule?
There are a million opinions about the best time to change your marijuana plant over to the flowering stage.
For those who don't know, your plant will stay in the vegetative or growing stage for as long as its daily 'dark period' is shorter than 12 hours a day. Or in other words, when the 'days' experienced by the plant are longer than 12 hours.
During this time, the plant does not produce buds at all.
In nature, as the days get shorter, cannabis plants will start flowering (producing buds) because they think winter is approaching.
The indoor grower will need to artificially induce flowering/budding in plants by changing the light schedule so the plant receives only 12 hours of light a day, and 12 hours of uninterrupted darkness.
Once the plant is changed over to the flowering (12/12) light schedule, there is generally another 6 weeks-5 months (average 2.5 months) before the plant's buds are ready for harvest.
So When is the Best Time to Start Flowering Your Cannabis?
The real answer is that i's a matter of personal preference and also depends on what end result you're looking for. There are two major considerations when choosing the right time to switch to 12/12, the age of the plant and the height of the plant:
Age: Some people feel that a marijuana plant which has been grown from seed will not produce as many buds or have enough resin production if the plant is not given at least 60 days in the vegetative stage to mature before it's changed over to the flowering stage. However, other people report success with germinating a seed, and changing the plant right over to flowering after only a week or two to get one or two sets of leaves. When growing with clones, age is not an issue and everyone seems to agree that you can switch directly to flowering once your clone has established roots. This is because even though the clone is small, it's still a 'mature' plant since it is made of a piece from a mature plant. In any case, whether you wait until your plant is 60 days old or start flowering it earlier, both methods can be successful, and you should switch your light schedule at the time that best fits your needs.
Height: A general rule is that your marijuana plant will double in size during the flowering stage from the point where you first change over the light schedule to 12/12. Some plants will grow more, some will grow less, but a good rule of thumb is to change your light schedule over to flowering when your plants have reached half of their final desired height. For those growing in a small space, height may be the primary concern. However, there are many techniques available to grow a short,bushy weed plant.
In optimal conditions, you would probably want to vegetate your plant for 60 days or more, and then switch it over to flowering. This will obviously give you the best final yields (and also the biggest and tallest final plant). However, if space is tight, then it's better to switch when the plant is half the final desired height, or even to just flower your cannabis plant straight from seed. If you're growing with clones, then you should only consider the height when switching over to flowering.
Tha-Tha-Tha That's all folks