For organic certification the time is 3 years.
One thing that affects chemical,leaching is how much water passes through the ground. If the area is well irrigated it will take less time for the purification process to happen.
Annuals should be OK in areas that have not been sprayed and have had good irrigation for a couple of years.
Perennials would be my concern. They will store the chemicals for a lot longer or forever possibly.
What perennials do you have growing that you are considering harvesting?