I've sorta changed my mantra on this over the years as I've learned to become more careful. If it's hydro, you can start as soon as you put the rockwool in the system. It just has to be at a very low concentration, like 100-150 ppm. I used to not add until they were like 3 inches tall, but now I do it right away and it's fine. You just have to be very careful. I think it helps to get the roots going faster and that's why I do it. I also like to use Thrive Alive w/ B-1. That really helps with the roots and acts as a very mild fertilizer for seedlings and cuttings. Soil is a different story. Don't start feeding until it is at least at least 3-4 weeks into growing. There are enough nutrients in the soil to take it out that long. Then do it like every 3rd watering or so. You just have to be careful and know your plants. If every 3rd waterisng is too much, then do every 4th. On the contrary if it's not enough, start adding more nutrients per watering, and if it's still not enough, then up it to every other watering until it starts responding. Basically, I'm just saying know your plants and watch them. That is the best way to do it, especially in soil. Hydro is a bit more exact so almost all plants respond the same (to nutrient levels that is). But I gave you some decent generic advice that should help you with whatever you're trying to do.