Once a banana is cut open, its shelf life decreases significantly due to exposure to air, which causes oxidation and speeds up spoilage. When left on the countertop at room temperature, a cut banana will typically start to brown and become mushy within 1 to 2 hours.
The rate of spoilage can vary depending on factors like ambient temperature and humidity. In warmer and more humid environments, the banana may deteriorate even faster. To extend the shelf life of a cut banana, you should refrigerate it as soon as possible.
If you don't plan to consume the entire cut banana immediately, it's best to store the remaining portion in the refrigerator in an airtight container or wrap it tightly in plastic wrap to minimize exposure to air and slow down the browning process. Even with refrigeration, it's advisable to consume the cut banana within a day or two for the best quality.