Daily Light Integral is the proper way to assess how much light you are providing. Read the wiki. Get the gist of it.
Rate x Time... one without the other is mostly useless.
A rate of photons (umol/s PAR rate) without hours of operation (time) is only 1/2 of the story. This is like asking how far you drove and only knowing the speed (rate) you traveled but not the hours of travel (time).
DLI also makes size of garden and differeinghours of operation irrelevant for comparison. 35 dli is always 35 dli regardless of garden size or whether on 12/12 or 18/6 light cycle. DLI is what matters as far as yield - relative to genetics of the specific plant you are looking at.
Some dark cycle is useful. To say 16h, or 17h, or 18h of light is best is just blind faith. I've never seen any proper research that'll distinctly say one cycle is better than another beyond the basics of 'a dark cycle is a good idea'. I use 16/8 just fine. Keeps a greater portion of my electricity use during off-peak hours (cheaper).
So, 35-40 DLI production from light is a good start / target.. then, adjust power/distance based on how the plant grows. If you cannot provide that much DLI over 18h, in some contexts going 20/4 or 24/0 will be better than providing less DLI. I dont know the specific point where that becomes true, but it is inevitably true at some point.
in short -- if you have an under-powered light it 'might' be a good idea to go longer than 18hours. If your light is sufficient, stick to 18/6 or 16/8 in vege. Don't go lower than 16/8 to stay safely away from unintentionally causing flower phase. Anything approaching 14/10 could induce flower phase.