I usually leave the fudge on the counter in a covered container and it will stay good for quite a while. There is so much sugar in fudge that it isn't an hospitable environment for bacteria. You can also place it in layers with parchment in between and store in the fridge in a sealed container. You don't want to leave fudge open in the fridge because the humidity can affect it. I also freeze my fudge. I place the pieces on a parchment-lined tray and freeze overnight and then place them into freezer bags.