PT is a pretty genaric term. There are different treatments for different purposes. All of them require that you research them and discover what the manufacturer says they will do. I use copper napthalate (brushed on) on posts I am setting in the ground...mainly for the insect resistance.
I agree with the statement that it is usually neglect that has brought on the demise of the timber. Protect it from the wet dry cycle and it should last forever.
