Down here where termites are a constant threat any untreated lumber won't last but a couple of years. My beds are all over 35 years old and made with the old stuff that has arsenic in it. Even with the arsenic some of my boards that obviously didn't have enough got eaten by termites but most are solid as can be but severely warped after all those years. I don't think the arsenic is a real problem any way since worms don't seem to mind it at all and tend to concentrate right up next to the boards. If it is poisoning me it is doing it really really slowly.
Bill