I'd flower them the moment they show sexual maturity- alternating nodes.
That's all the space you've got... and they will FILL IT.
I flower plants at 5-6" in one gallon pots and find that 9 per square metre makes for a dense, thick canopy within the first couple weeks of flower, and might even be considered slightly crowded if each plant doesn't keep to a single main lead.
Lookin' good.

Oh- if they are not feminized seeds, you can go another week because you will be removing 4-5 males and ending up with extra space- you don't have to worry about what appears to be intitial crowding, as 5 plants with an extra 2 week of veg time from where you're at right now take up about the same space as the 9 would have.