I recommend preparing the filling the day before you plan to eat the Korokke. If you want to make them further in advance, you can prepare the filling, shape, and bread them. These can then be frozen on a sheet pan. Once fully frozen, they can be packed in a container and stored in the freezer for up to a month. If you plan on freezing them, I recommend making 12 smaller Korokke so that they heat up properly when you fry them. One final possibility is to prepare the Korokke all the way through the frying step. These will keep for a few days in the refrigerator and while they won't be quite as nice as a freshly fried batch, they can be reheated in the oven to crisp.