12/12 Is used for Budding the plant
18/6 Is what most people use to Veg
24 is what some people use for starting and vegging
I personally use 24 for the first couple of days then switch to 18/6.
This will also depend on some factors. do your plants like 24 hour light? (some cant take it) Thats why 18/6 is a more general rule for veg. Also if you cant get your plants to start budding on 12/12 I have heard that you can put it at 10/14 or 11/13 untill they start to bud.
Switching to the 12/12 (Budding cycle) is up to you, unless the plant is an autoflower, You can grow it to 5 feet or 5 inches. Most people wait till they are about a foot and a half tall to start buding them, Keep in mind they will probobly double in size while budding. Hope this helped.