What is the shelf life of these things?
It depends on who you ask. All of Saddam's pre-1991 munitions were considered "degraded" by MSM types. However, our chemical stockpiles at Redstone Arsenal and other locations are still monitored and considered extremely dangerous. They're a century old.
If Syria possesses binary chemical munitions, the ingredients are kept separate until the weapon is deployed. Under proper storage conditions, the shelf life would be significant, if not indefinite.