Many factors to consider. If you are using a peat based soiless mix, I would only use veg nutes if it takes longer than 3 weeks to show sex. Even then, I'd start small at like 1/4 strength or something. After that, I would jump right to flowering nutes. Depending on the brand of nutes you are using, I would do something like 1/2 strength once a week. I find that autos really don't need all that much. If you decide they need more and want to go higher than 1/2 strength, work your way up slowly. I use Botanicare and 1/2 strength once a week seems to work for me with the strains I've grown.
the plants are at 2 1/2 weeks old. All the cotyledons have began to turn yellow with some slight yellowing on the leaves. My plant food says 2 oz. for 3-6 inch plant, so should I mix 2 oz with like 2 liter bottle and feed through soil