24/0 --> 12/12: After many grows I have noticed that running the lights 24/7 in the vegetative stage is of little use compared to 20/4 or 18/6. I also think that constant light without a rest period is not beneficial for the seedlings, puts your equipment under unnecessary stress and runs up your electric bill for no more than a 5-10% increase in weight, at the most. It can be said that this is unnatural but then again, periods of close to 24h sunlight during summer days is not unheard of in arctic regions. On the other hand, how would you cope with around the clock work and no sleep? The dark reactions are important for the plant to build up and move around essential biological components, ultimately it affects things like resistance to stress and sufficient root structure for further development. Furthermore, the 12/12 stretch becomes unnecessarily long due to the big difference in daylight hours. Hey! Where did the sun go?
20/4 --> 12/12: This is a kinder option for the plants, your equipment and your electric bill. This gives the plants some time to rest and your equipment time to cool down once and a while. I run the lights over my mother plants with this light schedule, the light is on a timer and turn off for an hour four times a day. Four consecutive hours of darkness are sufficient for the dark reactions but still, the 8 hour jump after the switch to 12/12 is a bit of a stretch.
18/6 --> 12/12: This is a valid option to the light schedule above. Not that big a difference in final stretch as the stretch caused by increased dark hours in the vegetative stage evens out with the lesser stretch in early flowering stage after the 12/12 switch. The plant might be a bit stronger under this light schedule compared to the one above as it has some time to rest. 18/6 is no where near the equatorial daylight length that sativas are used to but is suitable for both indicas and sativas during the vegetative stage.
20/4 --> 18/6 or 16/8 --> 12/12: This is a good option, at least when growing from seeds. This light schedule gives the plants a chance to prepare for flowering in good time with minimal stress. I run the lights 20/4 for about two weeks, then switch to 18/6 or alternatively 16/8 for another two weeks and finally 12/12 for the duration of the flowering stage (I usually drop the daylight hours to 11/13 in late flowering, this benefits especially sativas but works well for indicas too, don't go much lower though).
20/4 --> 18/6 --> 16/8 --> 14/10 -->12/12: The best option. Same as above but a little more work. Depending on the length of desired vegetative growth, decrease daylight by 2 hours every 7-9 days. If you are going to take the time to grow weed, why not take the time to reprogram your timer?
12/12 from start: This is good when flowering clones but can also be used on mostly or pure sativas. I always clone the strongest females and keep them in a constant vegetative stage in a separate grow room. Once my harvest is done, I take a clone of the mother plant and move it into the flowering room. Usually the mother plants have several months of vegetative growth as well as rigorous training under their belts, so these heavy duty bonsai plants are well established, strong and produce monster crops. Their stems are usually as
thick as my thumb when they go into flowering.
great share from weediologist 
