Donuts taste best when eaten just after baking. Ideally, when they're still warm, but definitely within the first 12-24 hours. If you don't expect to finish all 18 donuts on the same day, my advice is to keep the batter stored in the refrigerator and bake the remaining donuts the next day. If you do bake them all in one go, store any leftovers in a sealed, airtight container for maximum freshness!