Hi. first post here. I personally said 1875-1900, although I believe that Slavery would be ended completely at the latest by the last decade of the 19th century. Honestly, it really all depended upon how long the war lasted.
In 1863, Gen. Patrick Cleburne had proposed the idea of emancipation for military service, and although not very well received immediately, the idea gained steam the next year. There had been reports of free Black men as well as slaves seeking enlistment in the Confederate army virtually from the beginning. I have seen estimates ranging from 10,000 to over 60,000 Confederate soldiers of African descent by Wars end, but no exact number has ever been reached because those who served as cooks, teamsters, and "manservants" were granted full pensions as well as combat troops.
had the war ended in 1863, as it does in a piece that I am working on, I believe that at least a small minority would begin stumping for emancipation fairly soon after the war. Since the Confederate constitution specifically forbade Congress from acting upon the matter without a constitutional amendment, it would have begun at the State level. I would personally believe that Virginia, Tennessee, and Kentucky (If her secession was recognized at wars end) would have abolished the practice first. The practice would then have spread State by State, especially as the "millstone around their necks" as Jefferson Davis once said, began to be seen recognized for the immoral practice that it was. there would probably have been a couple of hold out States, which would have ultimately have to have given up the practice or secede themselves by the end of the 19th century.
international opinion was slowly turning against the practice, and eventually, so would the opinion of the South.