Dear Richard,
Pinholemaster is correct, it depends how you look after them, handle them carefully by the edges not between your fingers, never handle with wet / damp hands, never leave them on the darkroom bench, always return them to the packet after use, store them in a clean area not exposed to large temperature fluctuations, after that its a 'how much you use them calculation' obviously grades 2 to 3 get used the most, we would say for the average user they should easily last 3 years or more, I would personally replace after 5 years of use.
Simon ILFORD Photo / HARMAN technology Limited .