A buttercream cake can typically sit out at room temperature for up to 2 to 3 days, depending on the specific conditions. Buttercream is made with butter, which is more stable than other frostings and helps preserve the cake. However, factors such as temperature, humidity, and the ingredients used in the cake can affect its shelf life. It's important to keep the cake in a cool, dry place away from direct sunlight to prevent it from spoiling or melting. If you need to store the cake for longer or in warmer conditions, it's best to refrigerate it.