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.
Subject: Lawn Chemicals / Pesticides / Grubicides
Tried to quickly search for questions about lawn chemicals I had on the site but couldn't find any threads that dealt directly
Once applied, how long do lawn chemicals, pesticides, or grubicides remain in one's soil?
Indefinitely, a long time, or just a few seasons or years?
The extended use of pesticides on my family's lawn has probably been year to year quite sadly. I want to turn this around.
I was just reading a thread posted by CastyMcWrinkles Just Getting Started..., which Round Aboutly (hehe, ehh couldn't help myself ) dealt with lawn chemicals.
Is there a way to help the process of detoxifying one's lawn in the spring and helping the grass and soil?
The majority of my issues have been with grubs and moles, but you guys already helped out tremendously with those questions I had.