12/12 from seed

so im starting a 12/12 from seed grow and i was wondering if anyone know aprox how long it takes to sex the plants?

im using a 400w light and was planing on doing 24 1gal smart pots. should i give them a couple weeks under the mh bulb or shld i do hps all the way....?

any imput or advice is apreciated

roughly between two and three weeks!