Once opened, baking powder has a short shelf life. In the best of conditions, cool temperature and no dampness, it may last up to a year and a half. In less than ideal conditions, baking powder will last only 3 months.
To test the potency of an opened container of baking powder, mix 1/4 teaspoon baking powder with 1 tablespoon of water. If it bubbles, the baking powder is still good.