Seriously. I read some source that put the labor yield of American slaves at around $97 trillion dollars. Of course that's compounded at 6% interest because cotton commodities have certainly done that well historically (or it's more like 0.002% growth, but whatever). Considering we spend about $500 billion on predominantly-black welfare subsidies (cash/food/education programs), we should have them paid off in about 200 years.
Ironically if kept here to fester and assisted as they are (among other factors), the US doesn't have that long of a timeline ahead of it.