ive used all light schedules for veg you can think of,
18-6 promotes faster root growth compared with 24-0,
plants grown under 24-0 seem to stay shorter and more bushy compared with 18-6, i find i got longer buds when i use 18-6 or 17-7 for veg.
as for autos ive done lots of testing with several auto strains and produced my own pheno selected auto strain,
my findings was this,
autos grown under 24-0 from seed to harvest showed sex around week 2, they started flowering soon and reached harvest pretty quick, how ever yields where tiny such as 7 to 15g dry per auto, most autos are bred to be grown as sea of green, so lots of 56 day autos grown under 24-0 pays off, but only if you grow large numbers.
so using the same auto strains i had grown under 24-0 i tried under different schedules, my own experience has shown that my autos easily hit 2oz per plant when i grow them under 12-12 from seed to harvest, now im not saying 12-12 is the best schedule to use but im pretty confident that 24-0 is the best schedule for a fast harvest and low yields per plant
but if you want bigger yields from your autos then use 18-6 or what ever schedule gives less light per day.
what i think happens is this, autos dont flower and reach harvest due to age in days, if this was true then they would never flower under 24-0 from seed because they would not know what 1 day is without having a regular dark period every 24 hours, so we can rule out autos flowering due to age in days. so we nee to think of what else makes them mature and reach harvest. to me the thing i can think of is the flower and mature due to age in light hours.
the reason i got to this conclusion is that under 24-0 my autos showed sex at week 2 from seed, they started flowering 2 weeks later and reached harvest around day 60ish from seed.
under 18-6 from seed to harvest they showed sex around week 4 from seed and started flowering around week 6 from seed, harvest date was then around day 80ish.
under 12-12 from seed to harvest it dragged the grow time out even futher so they took longer to mature, longer to flower and weeks longer to reach harvest,
but my yields went from 7 to 14g dry per plant under 24-0 from seed to harvest and under 12-12 i was easily hitting 2oz dry per auto but the down side is it took longer to reach harvest.
so from the tests i done with the strains ive grown i found if you want a quick harvest then use 24-0, but if your growing just 1 auto then dont expect anything more than 7g to 14g per plant, but if you grow 1 auto and want a decent yield then give the auto less light hours per day, so 18-6, 17-7, 16-7 etc etc, pick one schedule and stick with it, i went down from 24-0 with poor yields and tried 12-12 from seed to harvest and was very happy with the 2oz i ended up with.
not all autos might follow these rules but i was fed up of getting really poor results under 24-0 and i kept listening to everyone say 24-0 is the best to use as it gives the most light hours per day, well i listened and got poor results, so i changed the schedule and i got bigger yields but longer grow times, so if anyone asks me which schedule to use for autos id have to say anything between 18-6 and 12-12,
maybe the 24-0 with autos slows root growth and this keeps the plant small where as 18-6 promotes root growth and allows the plant to get much bigger, plus when the light are off the plant is still growing,