You can look up the half-life of most such chemicals online. The generic name in a web search, along with "half life," should bring up some scientific papers, although unless you have a university internet account or similar, you'll need to stick to the ones that quote results in the open (sometimes the abstract is outside a paywall, but the rest of the paper costs money...some institutions publish all results under a free license, bless them).
Perhaps we can help you search, if there are a large number of them you're interested in.
If you spread a pound of something on three half-lives ago, about two ounces will remain. It's hard to say when it all has degraded or washed away, but most times a chemical will cease to be important in a fraction of a year to a few years, depending on the chemical. It varies widely.
"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.
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