It depends on use cases and the average accepted length I've seen is five year shelf life unopened and six months opened. Generally though if you can smell anything through it when using a product the concentration of bad stuff in the space is too high or you really need to change your filter.
I had my airbrush out from working on the weapons and decided to give it a shot for doing some weathering on the cod and butt pieces. I really loved how it was looking so it motivated me to get some additional weathering done today.
I am not ready to say that these are 100% done, but I am really happy with how they are looking!