18/6 is a minimum veg cycle light time, some growers like me prefer a 24/0 veg cycle. I like 24/0 because the plant is growing 24/7 until I go to 12/12 when they are ready to flower.

12/12 is pretty much the standard flower cycle some will do a 13/11 light cycle just to get a little more growth and speed things up, but going beyond 13 hours is not recommended.

JG