What causes a banana to turn brown and how long does it usually take?
According to Britannica.com, Bananas, like most fruits, produce and react with an airborne hormone called ethylene that helps to signal the ripening process. Bananas produce a larger amount of ethylene than most other fruits. While a banana in the beginning of the ripening process might become sweeter and turn yellow, it will eventually overripen by producing too much of its own ethylene. High amounts of ethylene cause the yellow pigments in bananas to decay into those characteristic brown spots in a process called enzymatic browning.
Do this fun experiment to find out how long it takes.