As Ian said, we do advise they are changed every few years ( we would say that would'nt we ! ) but its true. Quite often poor handling degrades the filters ( they are a coated product after all ) and of course it depends on how much they are used. I would say in normal use they should be fine for 5-7 years, after that probably best to get a new set, In saying that as long as they are not 'optically' degraded as long as you do not need the top end 4.5 and 5 its more likely that they could remain usable for longer.
Simon. ILFORD Photo / HARMAN technology Limited :