If you are growing it as a plant food, then the idea is to crop it before it puts up a flower stem. You can do this 4 times a year with an established plant. If you are growing it for the bees, then once it starts to fall over, cut the flowering stems down (not the leaves this time), and the plant will flower again. If you're on your toes you can rotate doing this so there are always some flowers. The cut-off flowering stems are best soaked to make a liquid feed, as if you chop them and use them as mulch, they have an annoying habit of rooting themselves in your veg bed.