I was told by a structural engineer that when they changed the way pt lumber was made due to toxicity concerns it greatly reduced its lifespan - his estimate was that it typically lasts 10 yrs before it weakens due to environmental conditions, and in areas like the northeast that have constant freeze thaw cycles and wet seasons he trusts it for about 8. If you want your structure to last a long time it may be wise to use on of the many kinds of post base connectors or brackets.

In the northeast we typically detail 12” of crushed stone, footing above, leave a couple inches exposed above grade or soil line min., and if possible taper the footing top to avoid water sitting against the post, doesn’t have to be too dramatic but just a soft pitch away.

Our frost line is 4’ so I’m guessing you’re something similar where you are.