The best answer? It varies.
Best answer is a shelf life of @10 years sealed.
If you use it, replace the filter.
How long it lasts has much to do with particulate size, microns or chunks.
Then there’s mask life, if you find any cracking when you stretch and move the mask, its suspect. And while it may stop certain things, it may well not stop the small dangerous bugs then.
There are some very good US made nato spec filters, I’d be looking into them I’d the masks good. Remember most places don’t surplus off good items.