I guess I'll throw my answer into the ring...People have chosen to use these times "18/6 and 12/12 because trial and error (which has been pointed out by many on here) has proven good results for most strains...Reason being your growing indoors...now once you accept that, you should know there's no possible way to really mimic outdoor style growing...The benefit of growing indoors is to create your own personal environment (however lighting cycle you choose, co2 you choose, nutrients etc etc...) getting back to the point here...these times are great because of the fact that you could never get sunlight for 18 hours outdoors, thats the whole point here, indoor growing is all about creating the most beneficial environment for the plant. Thus giving a plant more light than it could ever get..you'll never get 7 to 9 weeks of straight sunlight outdoors either lol...the 12/12 is a consistent light dark period to control exactly how your plant is going to be forced into flower and stay that way..sure you could give a plant 11 or ten of darkness but that will force maturity to the buds. Another quick point (tired of typing lol) is 18/6 after trial and error "which is in most grow books as facts" is the best ratio of light to dark to get the best root system...which in my opinion is by far the most important factor towards a plants health, size, and ending yield... 24/7 your plants will grow fast and lush but with the least amount of root growth, 18/6 will give a decent speed of foliage growth and root system, while 16/8 will give the most roots but a smaller foliage...at the end of the day its ALL about your own personal grow space and what your trying to achieve with what you have to work with....
