18/6 is standard, some people do 20/4 or even 24/0 but there is a lot of different schools of thought here.
Most people agree that plants benefit from a dark cycle or that the plant can only use so much light in a given day, so 18/6 is the most common.
Then there are those who don't think plants need dark period and that 24/0 maximizes potential. Some people are kinda on the fence and go 20/4.
Then there are those that mess around with different cycles. I've heard of people going 24/6 or 20/8 just to see. They also do strange flowering cycles or try to mimic the the sun and seasons by slowly decreasing the amount of light every day.