How long does Plasma treatment last?

This depends on the material and subsequent handling and storage. It can be from minutes for glass, and metals to many months for e.g. polypropylene. It is always better to print, paint, glue etc. as soon as possible following plasma treatment but the ‘relaxation’ of the surface is asymptotic and even after days the surface is often much more active compared with an un-treated surface.
The reason for material dependency is that most commercial polymers contain additives such as anti-oxidants, mould release agents and anti-block agents, which are oily or wax-like. These are incorporated to improve manufacturability and are designed to migrate to the surface.