Define 'never have lasted'. If you mean slavery would not exist in a free confederacy of today then you're probably right. Would slavery have gone on another 30 or 50 or 75 years? Quite possibly. The fact that the southern states went to great lengths to protect slavery in their state and the confederate constitution is an indication that they didn't expect it to die out soon.
I'd say around 30 years. I think the only place slavery really hung around for a long time was Brazil.
Of course, their are some Mid East countries where it goes on today (usually the Religion of Peace enslaving Christians), but not as a national institution.