Gas pressure in a closed chamber at constant temperature will last forever, unless leaking or chemical reactions occur. Examples are methane in underground gas reservoirs that are hundreds of thousands of years old. Gases stored in bottles. Pressure comes from the gas atoms (with velocity) colliding with the container walls and other atoms. No collision can be fully elastic and therefore some losses are expected.... as it is not a perpetual motion machine. What gives the gas atoms continued momentum/ energy over such long time periods?