You can check out the 4x4 journal in my sig to see how 4 plants filled up a 4x4 when flipped at 5 and 7 weeks-both totally filled up the canopy. If you have a lot of small plants you should flip when they're much smaller, waiting even 4 weeks might be too much depending on your footprint. It also depends on if you are growing from seed or from clones because from seed, the plants need to hit sexual maturity before they start to flower-clones are sexually mature from the get go, so you can flip them at any size and they will flower.