Since coco doesn't contain any nutrients, you can generally start fairly soon. When preparing the coco I'll precharge it with a very dilute solution to begin with. I don't like to water my seedlings very heavily so that usually gets it through its first week or so. Young seedlings don't need much in the way of nutrients. They mostly need some patience and time to develop a root system, then get them on a regular feeding schedule. In a young auto I'd start with 1/4 strength of your veg state nutrients plus a CalMag supplement and scale up ad it gets bigger or shows signs it's ready for more feed.