The first stone tools were used by Hominines 2.6 million years ago. Assuming we have evolved from Homo Habilis, then we have been eating meat for at least that long. The earliest evidence of control of fire is about 1.7 million years ago. So, we have been eating cooked food for at least that long.
 Oldowan - Wikipedia, the free encyclopedia
 Control of fire by early humans - Wikipedia, the free encyclopedia