The stuff to really worry about were the mercury based fungicides used in the 40s. We still use chloropicirin (nerve gas) to fumigate land. Methyl bromide is getting phased out, but man it was good stuff. Kill darn near anything. Weeds didn't even come up. The fumigants would burn the piss out of your eyes and make your nose run. You knew in a few minutes if the hose was leaking. Fortunately, unless you covered the soil with plastic, the gas in the soil was gone in about 3-5 days.
Like I said, my rule would be anything less than 30 days after application and I may be wary of using it unless I knew the chemical used, rate, etc.