I'd say focus on DLI, not specifically hours of light. Some strains do better with DLI up into the 40-60s, some do well around 30-40.
There is plenty of comparisons of plant growth per DLI, and the general consensus amongst commercial cultivators is that 18/6 at 350ish umol/s is the minimum needed to grow autoflowers and have a successful minimum yield, and 650-750 umol/s over 18hrs is the average amount of flux needed to maintain a healthy harvest over a variety of strain in controlled conditions. That's not to say that more won't mean better yields, it just means that yield efficiency usually drops off drastically after 700umol/s over 18/6, and factors like heat and humidity tolerance, genetic light tolerance and feeding schedules play a bigger role. For example you could get away with running 14/10 with the same light ran closer, for the same DLI but less electricity and heat over a day, but most would believe that to 14/10 is instantly getting less light.
I suppose, in an inflated way I'm trying to say that your 18/6 could be less DLI than a plant in full sunlight for 10 hours a day, or twice as much depending on the intensity. There is a reason why the horticultural industry uses DLI as a primary, and lighting schedule as a secondary to control temps and humidity depending on season and artificial control.
That's also a reason people stick to autos indoors If budget and space is an issue. It's easier to meet DLI with a cheap 120w quantum board over 18-20hrs than it is with photos. With 10-12hrs max in flower, you need a standard of 1000-1200umol/s to reach DLI, and a lot of cheaper boards wont reach those numbers unless you are at 10" from plants, where coverage becomes an issue. With autos you have the option of providing 600umol/s over 22 hours for a better DLI, yet far less investment in equipment. 600umol/s in achievable with a 100w quantum board at 18" in most cases, where as 1200umol/s over 11h will need at least a 240w quantum board in the same area at 14" to reach the same DLI.
