There’s no simple answer for this one. Probably more than two years. Some older termiticide sprays, like chlorpyrifos, can degrade very quickly. (Apologies for using the chemical names rather than product names – see later. Others such as bifenthrin and fipronil will last longer. Fipronil lasts a disturbingly long time in the environment. Imidacloprid is somewhere in between. Chlorantraniliprole is a bit new but should last at least as long as imidacloprid. 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. The chemicals will nearly always last a lot longer under your house than around it. With a chemical, you might get ten years or more service life under the house but find it fails in two to five years around the outside. Some poisons, like imidacloprid, can be happily sucked up by plant roots so that while you might have no aphids on your roses for a while. There’s always a risk the poioned-soil barrier mightn’t last as long as you’d hoped.
–You’ll note I used the chemical names rather than product names. There’s likely to be some variation between products that rely on the same chemical poison (=termiticide. or ‘active’) but have slightly different formulations or origins. Even the difference in the size of the tiny particles of poison can make a big difference. You can get your soil tested to see what’s there or you can rely on the experience of your pest manager to tell you when another dose is needed. Try not to over do it with applications as more is not necessarily any better but usually carries higher risks.