I was playing the 1775 campaign as the Brits, and pretty much destroyed the American and French forces and occupied/controlled all the objective and strategic towns by 1780, but the game just continued to roll along until the normal end point of 1783 (I believe).
I thought when I had captured and controlled all the objective and strategic towns, that the Americans would call it quits.....or maybe if I held them for 2-3 turns. But no, these "never say die" yanks (me being one of them) would keep recruiting 6-8 units every so often and try to take a village or frontier fort, but would get chased back into the wilderness.
I kept expecting another shipment of French to show up, and make this interesting.....and so this is why the game would keep continuing....but the French had had enough also.
So for three long years, we play garrison duty, and started attacks against Louisiana and the West Indies for entertainment sake.
In the future, I guess I could quit on my own, but it would seem like the game would check to see if all the objective towns and strategic towns are controlled, and if the army ratios are pretty imbalanced then the Yanks would call it a year....and send their taxes to England.