Well, from your “cold north east” location, you are probably going to need a second or even third brood box. Your bees will need to have enough stores to last through the winter (40-80lb of honey) before they have enough to spare for you. Please don’t think that one box keeps it simple. Depending on where you are, that may just ensure that your bees don’t survive the next winter.
As for when to add the Flow frames, the answer is the same as when to add any new box. You want the existing box to have all frames with fully drawn comb, and the comb 80% full of brood, honey or pollen, plus every frame completely covered with bees. If you add a new box too soon, the bees can’t defend the new space, and you risk the brood getting chilled too. If you are going to use 2 or 3 brood boxes, you use the same concept I just gave you - bursting with bees before adding another. @Anon has posted an excellent set of photos telling what you need to see before you add a box:
I understand that you are excited, but please don’t try to “force” your bees to work the way you want. They need to do it their way, or they won’t do well.