I usually fed them immediatly after germination and often after that.

I prefer an even ratio of N : P : K 1:1:1, 20:20:20 etc. I like to dilute the solution quite a bit and feed very often with a weaker solution, rather than less frequently with a stronger solution. When they are larger and more mature I feed them like they are tomato or cannabis plants, not at all like cacti.

Once they are established and are no longer delicate many excellent growers use a time release fert like osmocote.