24/0 and 18/6 are in effect one and the same.

Both are the most commonly used time for vegging.
Apparently, 24/0 is the easiest (as you dont need a timer) and it speeds up plant growth
but the 18/6 apparently increases root development.
either way, your plants will not flower until you switch to 12/12, you may see "pre-flowers" which occur naturally before flowering begins, but that will be after a good couple of months.