The cotyledon (First small pair) leaves provide the seedling with enough food for the first few weeks. I always start giving them very small amounts of nutrients when I see them start turning yellow, which takes a good 3 - 4 weeks after sprouting.
If you're soil is nutrient rich already. You may not have to add any nutrients for a long time.
Safest way to know for sure is to just wait until your plant starts showing signs of nutrient deficiency. Let it do it's thing. They will tell you when they need your help.
