Banana bread batter can sit for a short period of time before baking, typically around 30 minutes to an hour. This allows the ingredients to meld together and the flavors to develop. However, it's important to note that the leavening agents, such as baking powder or baking soda, start to activate as soon as they come into contact with liquid ingredients. Therefore, if you let the batter sit for too long, it may lose its rising power, resulting in a denser loaf.
If you need to delay baking the banana bread batter for a longer period, it's best to refrigerate it. Cover the bowl tightly with plastic wrap or transfer the batter to an airtight container, then place it in the refrigerator. This can help to slow down the activation of the leavening agents and preserve the quality of the bread. When you're ready to bake, remove the batter from the refrigerator and proceed with the baking instructions, adjusting the baking time if necessary to account for the chilled batter.
It's worth noting that overripe bananas, which are commonly used in banana bread, tend to oxidize quickly. If your batter contains mashed bananas, the color may darken during the resting period, but it should not significantly affect the taste or quality of the final bread.