Most sources say that the microgreens are ready to eat when the second set of leaves starts to grow -- that is, a set of leaves that is not the
cotyledon. Some of my salad microgreens are starting to do this!
Top L-R: Radish 2.1, Radish 2.2. Middle: Corn 1, Salad Mix 1. Bottom: Sunflower 2, Sunflower 1. Very right: cameo by Corn 2.2, 2.2, 2.3
Under the cut: bad /dark/rotated picture to see the height of the corn greens, since you can't really tell in the first picture: