I kept them around until they formed seed pods, and when the flowers looked mature enough that I could see the little black seeds
I lopped off their heads.
I use scallions all the time and have hated to buy them at the store the last couple of months. Now I realize that I should have planted more first thing in the spring to harvest throughout the summer. And if we continue to let some go to seed, we'll never have to buy scallions, or scallion seeds again. The best part is that insects and animals want nothing to do with them, and they don't need any fertilizing or pruning. They just grow. Easy stuff!