Pressure treated lumber is advertised as having a long life (some say 40 plus years) with direct earth contact. The manufacturers soak the lumber in chemicals that penetrate into the outside surface. The most vulnerable area is where the wood is cut. Any such ends require localized treatment, if they are to be buried. My experience is that, usually, this lumber does last a long time. I have some fence posts, I put in 20 years ago, that I have since dug up. All of these were still as good as the day I put them in the ground. On the other hand, a few times I have found pressure treated wood under decks, even posts resting on piers, where the wood had decayed. As with all of life this goes to show that there is the design, the plan, the odds and sometimes something does not turn out as was expected. It is out of the “norm”.
Pressure treated lumber, and how to deal with it, is not well defined by the Washington state (where I work) wood destroying organism laws. The wood is manufactured for ground contact, so having it touch the earth is not specifically a defect. At one point I asked the WSDA about this issue and was told this — Fact: Pressure treated wood in contact with the soil will last much longer if all soil contact is eliminated. An inspector should probe, if possible, all pressure treated lumber which has soil contact and, if it is decayed, then call it out that way. If no decay is apparent, an inspector might, to be helpful to the client, say that grading soil back from the lumber, or putting it up on a concrete pier, will make it last longer. This advice may, or may not, make sense depending on the design elements of the structure, say a deck.
Personally, as an inspector, here is what I do. If I see pressure treated lumber in a crucial role, and it is in contact with soil, then I call out for grading or removal of the soil. For example, I call it as a problem when I find pressure treated lumber down in soil and it is used as a post under the house or on a deck of any height. In that crucial role, you do not want to take chances on rot, the result of wood to earth contact and the moisture that brings to the wood.
On the other hand, if I am inspecting a house and find a couple 4×4’s, for support of a handrail, sunk into the earth, at each side of the steps from a modest porch or deck, I check the lumber at the ground level and, if it is fine, then I do not say very much. No grading is applicable, with the post sunk, and it seems to be holding up well. In my view, simple and non-critical outdoor uses, such as fence posts, are common and, down the road, the repair is easy to do and the area easy to access. The repair can be done by a workman who is affordable and no structural damage is likely to take place.