This is my first time growing leafy greens ever! I have 10 of these plants growing under LED lights. They were sown on Jan 9. When do you guys think I should start cut and come again harvesting?

I think you could start now, just take an outer leaf from each of the ten plants. If that's not enough, go around again. I think you could easily take 3 leaves from each plant if you can use them.

Beta vulgaris- isn't that beet greens?

It's a variety of Swiss chard, though the flavor is slightly different, and you can harvest them very small like that, just one per plant, but I usually let them get larger. If you're raising them commercially, and have a market for it, "baby chard" would sell for more, but for just the few plants I would grow, I'd rather leave them until they are fairly large. As with most of those Asian greens, these are cut-and-come-again types, and keep growing from the center, giving the name perpetual.

I tried growing some chard a few times indoors, but it was much slower than lettuce, or any of the Asian greens I tried. It does grow through the summer, only slowing in intense heat, but recovering after.