Keep them on the counter until they start to turn brown -- no more than a few days. If you put them in the fridge, they’ll blacken within a day or two. The skin will also blacken if you put them in the freezer, but the fruit inside will be safe to eat. To keep them around longer, peel them, dip them in lemon juice (to preserve color and texture), and pop them in the freezer. They’ll be good for a month or so.