Different strokes for different strains, but you can put a clone into flower as soon as it is ready for the real world. I have not done this but many have. If it doesn't want to begin the flowering cycle, it will anyway, it just may take a few extra days. You can harvest more often, and cut down on a lot of things you would otherwise need (like a MH light) using this method. Of course each harvest will not yield as much as if you let them veg a few weeks.