I think they're good until you have to cut one? Is that right? The treatment doesn't penetrate to the core of the post like the creosote does, so any cut that exposes the middle of the post enables it to rot from the inside outwards? Like it does with tanalised posts.Copper oil stakes / posts would be safer next to a water coarse guaranteed to 40 yrs better than creosote for pollution reasons.
I think that's the case anyway, and this photo would seem to bear that out:
