How long do herbicides and pesticides last?

Question

I am a greenie that is looking into "weeds" as a food source. How long do the effects of herbicides and pesticides last, until the lawn is "safe" for harvesting again?

I live in a community of mostly rental properties.

Answer 1

Most are put on at such a low level that after a few rains it is all gone. I like to think of in  ppm. some environmentalist like to talk in ppb. Most equipment is not that accurate.