a plant will usually show sex(preflowers) at about 4-6 weeks from seed under 24/0 or 18/6 lighting. there is no need to switch to 12/12 to sex the plant. it will show in it's own time. be patient, this is not a hobby for those who are in a hurry. it is much better to wait til a plant has preflowers and alternating nodes before switching to 12/12. preflowers and alternating nodes show the plant is mature and will be able to produce more, better quality bud when flowered.