Since it has already flowered and is setting seed, you can either pull the plants up now or wait to harvest the seed. The seed takes about a month to mature after they are full size. If you want to save the seeds, you collect it after the pods (they are not true pods though) turn brown. You need to be quick after they turn brown, though, because the pods will burst easily when they are mature and spread seeds everywhere.
I've never grown chinese broccoli before, but the plant is probably not very edible now. The seed stalks make it woody.