When starting from seed the general rule is to wait until you see alternating nodes (non-symmetrical branches). That indicates that the plant is ready to show it's sex and ready to flower.

If you plan on keeping these plants around a while i would suggest cloning for sex (take a few clones from each plant, mark each clone well so you know which plant it came from, once the clones root, put them in flower, in 1-2 weeks you'll be able to separate the girls from the boys).

If you are just trying to gain experience then wait until you see alternating nodes then throw them all into flower...sort out your males asap and let the ladies do their thing. When your good seeds come in you should have a good grasp on what is to come.

I personally like a longer veg time...anywhere between 4-8 weeks depending on my method (SCROG vs. LST vs. big ol' bush) I'm a big fan of a low plant count and i find that my long vegs yield more.

good luck.

Skeet