Dalgona coffee fluff is supposed to stay for a very long time if using normal sugar. I have not tried it myself with normal sugar, but have seen on the Internet people keeping it in the refrigerator or storing the foam in the freezer. I have tried various sugar sweeteners for Keto Dalgona coffee Topping and have realized the following: Using pure Erythritol will whip up the foam, but does not hold very long. The same happened with using the Monk fruit mixture together with Erythritol.  It just flattened very fast.  The best and longest-lasting foam for Keto Dalgona Coffee for my opinion are Sukrin Gold, Allulose as well as Xylitol. All three of them give you excellent results and the Dalgona Coffee topping stays beautiful for a long time as you can see in the case of our recipe post you can use the Dalgona coffee topping for almost anything you can.