You can do it at anytime but you want the plant mature 1st really. Wait until you see alternating nodes or branches that don't grow right across from each other.
how many plants what size buckets and what kind of light are you using?
for the most part you want the plant to veg into the grow space. but know that when you click it into bloom the plant will most likely double in size. a good veg ensures a good harvest with a strong root structure.
my plants are nearly a foot an half and the pots are small but a mate were saying that i need to put them in bigger pots and then they will do better is that write.