I usually wait for the first set of true leaves to develop to the point where the second set of true leaves is beginning to form. My reasoning is b/c once the true leaves form, the plant is undergoing photosynthesis, and it needs something from the soil soon. If the plants are in sterile soil starting mix, they need something to keep them going. I used 1/2 strength fish emulsion last year because it is very gentle on the plants and unlikely to burn. But I can't remember how often I fed. Some brands of fish emulsion may have directions for seedlings on the back. I got mine from home depot.