My 8 year old son asked me what the longest War in history was and I started in on low-level conflicts lasting centuries between the West and Muslims and France and England and between tribes in Eurasia - well he wasn’t buying.
I sum up his question really as :
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He wants to know what the longest shooting “hot” War was between two City States or greater…**
I guess a 1-2 year ceasefire in the hostilities is cool but not a break from say 1918-1939 if you follow that
If I’m not mistaken, the Wars of French Revolution and the Napoleanic Wars lasted from 1792 until 1815 (with the Peace of Amiens in 1802 being the only break in hostilities). I don’t know how Europeans regard the two wars as seperate conflicts or one big one, but as far as I can tell, it’s just one big war as seem by us Left-Ponders.
Not only was this a fairly lengthy period of conflict (lasting over 20 years), but it was very widespread, with the Americans getting involved twice, first during the Quasi-War against France, and later in the War of 1812 against the British.
The Thirty Years War has that beat, lasting an actual 30 years with pretty much no time off. The Hundred Years war is out of the running due to several lengthy hiatuses. I’m sure there’ve been longer conflicts but I’m not sure any of them would qualify as a single “hot” war between city or nation states.
The Vietnam War(s) lasted from 1946-1975, and most of that time involved people shooting at each other- First the French vs the Viet Minh, then the Viet Minh/Viet Cong/NVA vs the US & Australia. We all know how that turned out…
The Chinese Civil War lasted from 1925-1950, a disturbingly long 25 years of pretty much constant shooting, bloodshed, and general unpleasantness.
Remember kids, never get involved in Land Wars in Asia! 
Also, the Sengoku Jidai (Japanese Civil War, “The Age Of The Country At War”) in Japan lasted for something like 150 years- with near constant military conflict the whole time…