If you can somehow kill slavery before the cotton gin is invented, there won't be a civil war no matter what.
There's always the possibility that South Carolina decides to go through with its threats in the Nullification Crisis or some similar shitstorm indirectly caused by its interests as a slave state and has to be brought kicking and screaming back into the union. Maybe that counts, maybe not.
Latest can be decades after OTLs Civil War. Secession doesn't gain the support of as many states and the South loses quickly due to the changes in warfare towards the end of the century. Again, it might not count.
The Civil War was preceded by a decades long Cold War of representation, economics, and demographics over the slavery question. Any point where the South could begin to lose, like the election of 1860, is plausible.