When is it safe to start using the grow big on my little babies??
As usual, it depends on a whole range of factors, none of which you've mentioned, so it's impossible to give you an answer.
I assume you're growing in soil - what soil did you use? How many times have you repotted? When do you next intend to repot and into what kind of soil? In normal circumstances, there should be sufficient nutrients in the soil for at lease 3-4 weeks growth - but it depends entirely on the quality of soil you used.
You can cause far more harm over-fertilising your plants by incorrect use of nutrients than you can by under-fertilising them.
Nutrients are supplements to soil that the plant has either used all of the available nutrients in, or has had those nutrients gradually leeched out of the soil by frequent plain water feeds. When there are no more nutrients available in the soil - that's the time to start using them.
