it depends intirely on your grow medium. Im using organic soil/perlite and i started using half strength nutrients after a week. You should chek out the nutritional value that your grow medium holds before any disicions though.
How old are the plants? You should not be in any rush to feed your plants.. wait at least 10-14 days after germination.... If you are using MG soil, you can wait 2 months...
When you do feed... you need to dilute... heavly dilute your nutrients (maybe 1/10 regular/reccomended strength)... the most important thing you can do for you plant is THIS...