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Assume you sign peace when you just have gotten 100% ww in republic (121wwp) and keep out of his territory. Then you will lose 8 wwp the first turn, 7wwp the next. It will take 19 turns to get down to level 0, and 43 turn until the war is totally forgotten.
For each turn you stay in enemy territory, or he stays in yours, add one turn. Unless the wwp is reduced to 30, or you are lucky with roundings.
List of turns and wwp
Turn 0- 1 reduction by 8: 121, 113
Turn 2- 3 reduction by 7: 106, 99
Turn 4- 7 reduction by 6: 93, 87, 81, 75
Turn 8-10 reduction by 5: 70, 65, 60
Turn 11-15 reduction by 4: 56, 52, 48, 44, 40
Turn 16-19 reduction by 3: 37, 34, 31, 28
Turn 20-23 reduction by 2: 26, 24, 22, 20
Turn 24-43 reduction by 1: 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0