The time it takes for septic root killer to work can vary depending on the product's formulation and the extent of root intrusion. In general, root killers may take anywhere from 2 to 14 days to effectively kill and decompose the roots within the septic system. During this period, the root killer works to inhibit root growth and gradually breaks down the intrusive roots, allowing for improved drainage and preventing further clogs. It's essential to follow the manufacturer's instructions and be patient while waiting for the root killer to take full effect.