Leftover roast beef can typically be stored in the refrigerator for 3 to 4 days. It's important to store the leftover beef in an airtight container or wrap it tightly in plastic wrap to prevent it from drying out or absorbing odors from other foods in the refrigerator. If you won't be able to consume the leftovers within that time frame, you can also freeze them for longer storage. When freezing roast beef, it's best to wrap it tightly in plastic wrap and then place it in a freezer-safe container or resealable freezer bag. Properly stored, frozen roast beef can maintain its quality for up to 3 months. When reheating leftover roast beef, make sure to heat it to an internal temperature of 165°F (74°C) to ensure that it is safe to eat.