When to flower???

You can induce flowering at any time when the plant is sexually mature (~5 alternating nodes) but be prepared for a weak harvest. I would wait minimum of a month +

 

Earliest and best are two different questions. Opposites, in fact, because the longer you veg the bigger bud sites you will be developing. Bare minimum is probably around 4 weeks of vegging but the yield will be pretty small for the effort, better let it bush out for 6-8 weeks total. These times are assuming from seedling not clones, which can be ready quicker.