The general rule of thumb for leaving completed cake batter before baking is not more than 1-2 hours at room temperature. Ideally, you should bake the cake as soon as the batter is ready. Leaving it for too long can affect the final texture and rise of the cake.
When you mix the wet and dry ingredients, the leavening agents (such as baking powder or baking soda) start reacting immediately, creating air bubbles that help the cake rise. If you let the batter sit for too long, these air bubbles may dissipate, resulting in a denser and less fluffy cake.
Additionally, certain ingredients like baking powder have an active period, and their leavening power may decrease over time. This could lead to an uneven rise in the cake or affect its overall structure.
If you need to store the cake batter for a short time before baking, it's best to cover it tightly and refrigerate it rather than leaving it at room temperature. Refrigeration can help slow down the chemical reactions and preserve the leavening power of the ingredients.
However, the best practice is to plan your baking schedule to minimize the time between preparing the batter and baking it. This way, you can ensure the best possible outcome for your cake. If you have to wait for a more extended period before baking, you may need to consider making the batter in advance and refrigerating it until you're ready to bake.