Question about using jax321

I I just started for autoflowers. They are about 2 and 1/2 weeks old. I transplanted them into a mixture of ocean forest and Happy Frog and the other two were transplanted into shores of Maine Stonington blend. I’m testing to see which one does better. When can I start using Jack’s 321

Monitor runoff PPM and begin feeding when runoff PPM is < 1,000. It should take 4 to 6 weeks from your most recent transplant for the nutrients in soil to be depleted to < 1,000.