You can swtch to 12/12 to sex your plants. It usually takes about 10 days or so, depending on the age from seed of your plants. If you wait for the plant to reach 60 days from seed it will usually preflower under 18/6. A general rule of thumb is that if you put them in flowering for 10 days you will have to put them back into veg for the same number of days before cutting for clones or reflowering, to prevent hermies.