If you start timing from when the banana might be considered ripe enough to be edible, my experience is that you have about three to five days before you should consider making banana bread. If you refrigerate a ripe banana, you might extend that by a week or two.
Also asked, what makes bananas turn yellow?
High amounts of ethylene cause the yellow pigments in bananas to decay into those characteristic brown spots in a process called enzymatic browning. It seems like your grandma was right—a green banana that's placed in a brown paper bag will ripen faster because of all the ethylene trapped inside.
How quickly can you ripen a banana?
Place bananas in a brown paper bag and close loosely. Ethylene will build up and circulate within the bag, speeding up the ripening process. Check at certain intervals in order to take them out at your desired ripeness. #SpoonTip: Place a ripe fruit such as an apple or tomato in the bag as well.
Why do bananas go bad so fast?
Storing produce together can speed up the ripening process. Plants naturally produce a gas called ethylene, which causes them to ripen. This will cause the bananas to turn brown faster because the ethylene hormone will be unable to escape from the air surrounding the bananas.