When to flip to 12/12?

It really boils down to how penatrating your light is and how much space is required between it and your plants to dictate what your max height is. Many strains double height during the stretch so you'd want to flip your light at half that height at the latest. If you're wanting to keep them shorter, give them all a little topping so they grow more bushy and train them out horizontally- maybe SCROG them under a net. It won't take very long to fill out your canopy with 6-7 plants, 4 plants would be less crowded.

When they have filled about one third of your available vertical height/space. Considering how crowded they are, you may be better off flipping them sooner. In that area, one or two plants would be better.

It depends of how much height you expect 1 month of vegetative phase is enough to have a nice plant but if your grow tent is small with 3 weeks you are ready to go to flo phase.