Megs say their products have a shelf life of 3-5 years but I have some of their products that are over 5 years old and still work fine. An old bottle of scratch-x is one of them. I keep all my products in a heated garage that is around 50-60F year around.
just make sure to keep it out of extreme cold or heat (try to keep at room temperature) and it should last 3-5 years or maybe even longer. giving your product(s) a good shake once a month won't hurt either...