boil a small amount (two teaspons worth) into some water and add that to your res... you are hydro, i'm assuming. the next week, double that, and the next week, 1.5x that. if you try that, look at them 24 hours after you've added it to your rest and look at the leaves, buds, etc. and see if any harm has been done by possibly adding too much. i love the stuff, but make sure you use non-sulfurized (non something, anyway... look at the threads) or you can have some serious problems on your hands.

guess to answer the question... i start using a little bit as soon as i start gettin the white hairs and as time passes, i add more.