when the takes dry up. on commercials if the waters still cloudy, then the fish are still feeding - they rarely stop, even in winter. on "proper" waters, keep an eye on your spots and if the spots are still cleared out, keep putting bait into them. if its sitting there for days, then cut back.
depends on how accurately you are baiting as well. chuck a kilo of bait into your garage and you'll hardly notice it. chuck it on your bedchair you will. see what i mean?