How long does it take for Zero to kill weeds?

It depends. Zero has to translocate through the plant's system and travel down to the roots. The process of translocation takes more or less time, depending on the current growing conditions. A plant can die within a week, or it can take several weeks if conditions are cold, dry, or very hot. If you are after some quick reacting product, try Yates Zero Rapid 1-Hour Action Weedkiller.