That also depends on the strain of plant and type of soil/medium you are in. It is something that you learn by experience to judge when the plant is ready for feeding. The key is understanding that when a seedling first emerges, it has the 2 little round leaves to use as food while its roots develop. That is a critical time because those tender roots can be easily damaged if strong nutes are added too soon.
I watch my seedlings for vigorous growth. The seedlings will always start out slow and then when the regular leaves develop they will start growing faster. That is typically in the 2nd week and that is when I will water with a light solution of Root66 to get those roots growing good. By the beginning of the 3rd week, they will start taking off and that is when I introduce them to a light feeding of regular nutes (this is when done in hydro or soilless medium that supplies no nutes).
If yer in regular soil or organic soil that already has some level of nutes in it, you don't want to add any extra nutes until they start asking for them, and that depends greatly on how you have them set up. My organic plants were started in Rooter cubes from advanced nutrients(any of the better quality starter cubes will work). These have no available nutrients in them and allow for the early root development without the risk of burning them. Once I see roots popping out of my cubes, I put them in small containers of light nutrient organic soil mix. Once they have really established themselves and are growing vigorously, I will transplant them into bigger containers (2gal) of more strongly organically nuted mix for their final homes.
I hope this helps as there are many different ways to do this and a multitude of variables that will affect your final methods of growing. You will find that just about everyone does things a little differently, and you will develop your own style that fits your grow environment.