All Menards 6x6's are .60 retention treated, which will basically last
forever in the ground. If its a 4x4, which is probably only .4
retention treated, it will last only a fraction of forever. But your
biggest problem will be sinking and heaving. Without a footing, this
post is going to move up and down, compromising the structure and the
tree. The hole should have been at least 42 inches deep with a
concrete base of at least 12 inches around. It would not be hard to
dig it up and redo the whole thing, but it will probably not just fall
over one day; it will just get weaker slowly.