I like to do it when the seed cracks and just the tip is sicking out like a little nose.
Some like to do it when the roots longer but I feel that's a bit more risky for damaging it.
Good Luck!
Hey Growmie,
It's recommended to transplant your seedlings as soon as the root emerges or once it's about 1/2 inch long. Hopefully this was helpful and I wish you nothing but success on your future endeavors.