If you noticed, there is a period life symbol on every box of perfumes.

http://en.wikipedia.org/wiki/Period-after-opening_symbol

as long as you want,but gotta keep it in the right condition,always in their box,out of sun light or flourescent light,not in a hot place/room,no bathroom.i still have about 10 discontinued gems i keep for more than 10 years,2 of them more than 20 years and they still smell the same.but i noticed those thats very synthetic smelling type will get spoilt easier,the opening notes especially

I don't think I have seen any in departmental stores.