Looks to me like you can start flowering pretty soon. Maybe in a week. Those are seedlings right? If so they'll stretch more than clones.

Many times you hear people state 6 to 8 week veg for seedlings. I've never been able to veg that long as I would have huge plants.

At any rate flower as soon as you want but remember you should let them get to at least 8 to 10" for any reasonable yield.