I start feeding seedlings right away with calmag and rapid Start.  Within 3 days I start adding microbs and my base grow nutrient.

From there when your ready for transplant from solo cups to big boy pots, everyone starts doing what they think is best for their plants. I feed a lot of shit to my plants but good shit and I do it slow this way I see if I'm giving to much or I need to give more.

When mature my plants get fed 6x a day so your plants reaction will be quick.