Depends on how you store it. Stored in the right condition some could last up 3 years or more. They will still lose some quality over that period. Humidity and temperature variation are the biggest enemies so they will be ideally stored in an environment which is of optimum humidity (I think it was about 60% at around a steady 20oC. Some use wine fridges to achieve this.
I have stockpiled and now finding some beans that I bought in 2010 have lost nearly all their flavour. A humid summer in Sydney seems to have had an effect as I was consuming 2009 beans last year that werent nearly as flat.