I'm not sure that there IS a written rule of thumb, but this is what has always been shared with me in gardening circles:
If you don't know if they have been treated, go TWO growing seasons before assuming anything is safe.
When I moved into my house in Michigan, I was super excited that we had some fruit trees and I was a gardening fool up there. Had a veggie garden, a few fruit trees, a few fruit bushes (grapes and blue berry) and two beautiful flower gardens. However, I waited two years before I fed any of these things to my animals because I did not know if the previous owners had treated them with anything.
There are all natural pesticides and fertilizers that one can use of you have a huge pestilence problem.
~*~ Val ~*~ Passionate = one step down from crazy
We can only be said to be alive in those moments when our hearts are conscious of our treasures - Thornton Wilder
We are what we are, no matter what we might wish to be, or pretend to be. - Dean Koontz in The Face
My favorite color is orange - I'm the normal one.