When left to their own devices, plantains can take up to several weeks to ripen fully. This time becomes even longer if you keep them in the refrigerator or another cool place that delays the ripening process. The good news about this is that you can eat plantains even when they are green --- but they are more of a starch than a fruit in this state. The less fortunate news is that, while you can speed the ripening of plantains, you will still need to wait a week or more. Bananas, on the other hand, ripen significantly more quickly; you can speed their ripening process in the same way.
Put your bananas and plantains into a brown paper bag that is large enough to easily hold all of them. The fruits should have plenty of room, rather than being packed or squeezed together.
Add an apple into the paper bag. Apples give off ethylene gas, which speeds the ripening process of both bananas and plantains. Fold the bag loosely shut and store it in a warm (not hot) place, such as on top of your refrigerator.
Check on the bananas and plantains daily. Your bananas are ripe when they are deep yellow with a few brown spots. Plantains, on the other hand, are not fully sweet and ripe until they are almost completely black. They should also be somewhat soft, though not overly squishy, rather than firm and hard as they probably were when you purchased them. Bananas should ripen in several days using this method, while plantains may take a week or more.
Don't forget that you can use plantains at any stage of ripeness. If you wish to serve them on a certain day, but they are not yet fully ripe, consider cooking them or using them in a different way that takes advantage of their starchy qualities. You can refrigerate your plantains and bananas once they reach your desired ripeness to keep them at that point. Do not refrigerate them before that, however, as doing so will halt the ripening process.