Switching to Flower?

ok a tent roughly 4.5'x8.5' and 7' tall with 2 600w hps lamps.......
21 plants in 3gallon potts i think,,,,, The girls are clones say roughly 5 weeks of hps veg time very healthy about 2ft tall measuring from the soil (not including the tall potts)

so i figure i have about 3 ft left at the most if i take in to account the space needed for the lights roughly 12''

so may question is when should i switch to flower?
 
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