Well, you could use the NOP standard... to be certified as organic farmland must be free of most pesticides, fungicides, herbicides or fertilizers (there is a list of acceptable 'natural' treatments), for at least 3 years.
Several people on this forum has espoused two months as a "safe" amount of time to wait before harvesting from plants that has been treated or sprayed. I think this is guesswork, as the amount of time it takes for insecticides & chemicals to work their way out of land or a plant is longer than 2 months, as evidenced by the watered-down organic standard of the NOP.
I wonder about all of this concern about home-use chemical weedkillers that may or may not have been sprayed on lawns... are these people also concerned about the chemicals in and on the produce they buy? are they buying entirely sustainably produced certified organic produce for their uros?
I advise you to use your best judgement, but if you are a worrier, you can send a sample of your soil to a lab.
--benedita