I always wondered how long baking powder lasted since I rarely use it. I found this useful piece of info.
Baking powder should be kept dry in a sealed container to prevent it from losing its potency. To test it, pour 50 mL (1/4 cup) hot tap water over 2 mL (1/2 tsp.) of baking powder. If fresh, it will bubble actively.
If this doesn’t happen, discard and buy a new supply.