Also known as baking powder or baking soda. Leavens should not be let out to sit for more than 15 minutes after adding it to a wet ingredient (such as the batter). Leavens don't really work after 30 minutes of sitting in batter. Leavens create carbon monoxide (C02) when added to and mixed in the batter, it then expands the air bubbles which were already created when creaming butter and sugar together.
If you are unsure about your leaven being spoiled, here is a great way of testing them to be sure it is still good.
Baking Powder: stir 1 tsp baking powder into 1/2 cup of hot water. The water should double vigorously.
Baking Soda: Use 2 tbsp vinegar and 1 tsp baking soda, if it starts to create tiny air bubbles and air mass then you know it is still good.