Is it safe to eat leftovers from a frozen entrée?

Yes. Family-size frozen entrées often aren't consumed in a single sitting. Leftovers are subject to the same rules of handling and storage as other cooked foods. Refrigerate leftovers in covered containers within two hours of cooking. Reheat the leftovers to 165°F before eating them.
 
Depending on their content, some leftover entrées will keep longer than others. Those that are preservative-free generally won't keep as long. Because they are acidic, tomato-based sauces generally will keep longer than sauces made of cheese or other dairy-based products.
 
Look for signs of spoilage such as mold. When in doubt, throw it out.
 
Source(s):
University of Illinois "Food Storage, Food Spoilage, and Food-borne Illness"