the usual is to go 18/6 for veg and 12/12 for flowering. You could run a perpetual 12/12 cycle, just remember to start seedlings when the older ones start flower.
Just remember when you do start a 12/12 cycle to make sure you have time to get all of your "work" done durring lights on. I didnt consider that on my first grow and found myself having to get up early b4 work to tend the garden insted of getting much needed sleep!
I think you're asking when "Bob", or you, can flip from vegging to 12/12? If so, the answer is whenever you want -- some folks go straight to 12/12 from seed, most veg for at least a couple of weeks, up to a couple of months. Just remember that the more you veg the bigger you plant will be, so patience is a virtue. Also consider techniques like LST to really maximize yield in a small space very easily.