I would have to say there is one way to solve this.
Answer: There is no true long lasting period of peace for any nation, civilization, people, culture, race, etc that had no conflict of any sort whatsoever.
Even during times of total peace, especially in Europe, there were usually underhand tactics and hidden schemes in the works.
War has been a constant throughout humanitys history, and I would find a better question to ask would be what is the longest stretch of time that NO ONE on earth was at WAR with anyone else.
That would be an interesting question to answer.