@SeaMaiden - It really depends on what you're using the baking soda for; if you're just using it as a cleaning agent, the age doesn't really matter, but if you're using it in baking, your results will not be as successful with old baking soda. You'll get a lot more rise/lift in your baked goods if you're using active, fresh baking soda, so I'd recommend checking it every 6-9 months to make sure you're getting a strong fizz when you expose it to vinegar (it reacts with acid to produce its leavening).
As for baking powder, I always check mine every three months since it loses its effectiveness much more quickly than baking soda and makes an even bigger difference in baked goods when it isn't fresh.