Probably true. I was being very conservative. More like 10 years in reality.
One point to remember is that slaves are only good for unskilled jobs as you have to force them to work agains’t their will. If you put them on a skilled job in a factory they will sabotage it at every opportunity. The more industrialized the country the less jobs slaves can do.
Even though he was a mere boy at the time, Booker T. Washington makes this and some other very interesting observations in his autobiography Up From Slavery, an incredibly good read even 130 odd years later.