Not generally. If high-quality and stored properly, they can last a while. (Sarin, for example, can last as much as a year). If left out in the open with other munitions, the chemicals tend to break down and become unusable as weapons (i.e. still poisonous...but when you detonate the shell, it releases a toxic sludge, not a gas. Not an effective weapon.) Generally, after '91, Iraq couldn't store them properly - storage sites for chemical weapons tend to be big things that are easily noticed by inspectors.
The US and Russia had long-term storage, too, and had better quality control and added additives to increase stability. Iraq's CW program developed along with the Iran-Iraq war...hence, stability was less an issue since weapons went from manufacture to use in a manner of weeks during the war. I'm willing to bet Iraq never truly stockpiled large quantities of chemical weapons.