I have at least a few pounds on hand at any one time, 4-8oz of any one variety. Its nice to set yourself up this way, my experience is that the hops stay good for a couple of years at least.
I have the advantage of buying from a supplier that buys in bulk and resells retail (2,4,8oz) at reasonable prices. It gives me access to stuff that is occasionally unavailable via other sources. I've got Amarillo and Simcoe in the freezer right now for example.
Whay are these hops in short supply? And Centennial? Thats an even more "standard" hop isn't it?