plastic is pretty durable as long as it is not exposed to UV light, unless it is UV resistant. also, heat can dry out the plastic. if the plastic is in the dark and not often exposed to temps over 90 degrees, it should last practically forever. if the boxes are installed in an attic, i would expect to see some deterioration in 20 years or so, but i don't think it would be a safety hazard. i don't think the boxes will ever just fall appart by themselves.