It really depends on the plant and the light cycle. Switching them to 12/12 light can make them show sex quicker but they can show sex in veg too. My shortest times were 2 weeks for 12/12 from seed and 2 months in 18/6 from seed.
Kludge is right on, it's mostly genetic how early a strain will show sex I have had plants show sex at 4-5 weeks veg, but this is pretty rare most will take around 2 months or more in veg or 1-3 weeks in flowering before showing signs of sex.
If their a bit older, not seedlings, throw them into 12/12, a week or so later you'll see sex. You then can either take cuttings, put them back in veg or continue to flower.
If their a bit older, not seedlings, throw them into 12/12, a week or so later you'll see sex. You then can either take cuttings, put them back in veg or continue to flower.