If you have leftover Brussel sprouts, you should throw them out if they've been at room temperature for more than 2 hours. If not, you can refrigerate them in an airtight container and use them within 3 days. One of my favorite ways to prepare fresh sprouts is to fry them with bacon, a pinch of sugar, and some cayenne pepper, but leftovers (if there are any) don't tend to last more than a day because they're so tasty!