There's no simple answer for this one. Probably more than two years. Some sprays, like chlorpyrifos can degrade very quickly. Others such as bifenthrin and fipronil will last longer. Imidacloprid is somewhere in between. The rate at which a poison degrades varies enormously over short distances. The main factors are: the initial dose applied, temperatures, rainfall, soil type, alkalinity, presence of plants, applicator skill and the chemical properties. Then there's disturbance from floods, gardeners and burrowing animals.