How long does leftover beef keep once it is cooked?

It can keep 2-3 days if you wrap it tightly in plastic and refrigerate it. It will be safe and unspoiled that long, but it will develop rancid “warmed-over flavor” in about 12 hours, so cooking a large quantity planning to use the leftovers for meals later in the week may result in throwing out the product because it tastes and smells rancid and stale.
 
Source(s):
Wikianswers.com "How Long Does Cooked Meat Keep in the Refrigerator?"
Susan Brewer, Ph.D. University of Illinois, Department of Food Science and Human Nutrition