We are in the process of replacing my mother deck that was originally built in 1984, all of the support structure is original including the ground-contact 4x4 posts. Now I will admit it's rotten and needs replacing but it's going on 39 years old. All wood is basic treated southern pine. A deer stand built with treated pine and painted from time to time should last 20 plus years as long as it's designed to it doesn't catch and hold water in any spots. I've built about a half dozen and never had a bit of trouble with any of them.
I always built 4x4, once you go beyond that lumber becomes exponentially more expensive if you are building several and cost is of any concern.
