The "battle" of Verdun lasted 10 months; it was less of a battle in the usual sense of a single let's-fight-and-loser-runs-away event, and more like an open-air industrial blender made of shrapnel and bullets into which a continuous stream of mostly innocent people were ordered to walk over a long, long period of time.
The Somme was similar, but worse because it was bigger. All war is hell, but World War 1 was much, much worse.