good 'ol boys say 30 yrs, then you turn 'em upside down and they go another
30. But of course, the tighter the growth rings, the bigger the piece, your
climate, and possibly seed stock of the trees are variables.
I have seen a five inch (heartwood) log after 10 years on the ground in much
better shape than a PT 4X6 on the ground perhaps a few years less.
-Rick