what size is this grow area?

i would tranplant and flower now, if you wait they just get bigger and bigger to the point you run out of room, a flowering plant will double if not triple in size in just 3 weeks so if room is a issue you better flower now.

you could wait for preflowers but again your wasting light on males and males will usually show sex first within 1-2 weeks into flower, like i said flower now and weed out the males and if you have tons of extra room left switch them back to 18/6 for a few more weeks than you can flower.

the choice is yours