pretty sure there is no difference when it comes to flipping the switch from 18/6 or 24/0. The plants are going to flower how they want to regardless of light cycles(faster from 24/0 than 18/6 in my experience),. changing your light cycle a week before flowering will make no difference. 50 day strains are few and far between, not saying there isn't any, but 50 days with Sub's 3D seems a little quick. All kinds of factors could be slowing your grow down, temps, nutes, ect. ect.
In many ways it is a matter of opinion. Also every person runs their garden differently, so the same strains are going to react differently in different systems. I have talked to growers and read threads where people say 18/6 is better for the plants, some people say it makes no difference so use 24 hours of light so the plant continues to make new growth.
Here are my feelings on the situation. I give my vegging girls 18/6. I feel they need at least 6 hours of dark, this gives the foliage a chance to rest, so for the next 18 hours it can produce at maximum capacity. Plants are living organisms and most living organisms need some rest to regenerate their strength and energy. In addition to this, there is a belief that root systems do their most growth during dark periods while the foliage rests. And as most of us farmers are aware stronger, healthier, and bigger root systems produce bigger, better buds.
Plus I have noticed with my grows that when I give the plants 18/6 through veg, the transition into flower seems to be smoother and quicker.
Again these are simply my observations and feelings, so you may take them for what you will.
