I have 4 new in box Mopar shocks for my 2002 Dodge 2500 diesel truck. The date codes are:06/23/2003,11/28/2003,01/08/2005,and 08/16/2007. These are gas shocks and have never been used, my question is how long does the Nitrogen stay sealed in the shocks before it naturally leaks out even though they have never been used?