Advice for Harvesting greens:
Pull plants only when thinning. Otherwise, snap off leaves, leaving the growing tip to produce replacements. Leaves of 3 to 4 inches in length are tender enough to use in salads. Stringy stems are usually trimmed off larger leaves before the leaves are cooked. Light frosts don’t bother mustard; harvests can continue until heavy freezes wipe out the plants.
I have noticed fast growth when I harvest them. To keep them longer I cook them for 30 minutes and then put them in a freezer bag.
Currently my plot has exploded with growth and hopefully I will have enough to harvest for myself and the local food pantry. Below is pictures of how my bed has grown: