This is a topic that has come up many times here on the boards over the past few years. I have done many side by side studies myself same clones same mediums and when doing compariosn of 18-6 , 20-4, and 24 hours on everytime the 24 hours light has out grown and ended with higher yields in my runs everytime. I have done this maybe 10 times and the results always the same the 24 hour on light plants grow faster bigger and yield higher then the others with 20-4 sewcond and 18-6 having bigger leaves but slower growth and least yield. so based on my own study I always use 24 hours light until flower. Saves me time as the plants gfrow quicker so less days of light needed