How long does Simon Legree expect his slaves to last? Why does he treat them the way he does?

When a man asks Legree how long slaves usually last, Legree answers,

Oh, about six years for the strong ones, two or three for the "trashy" ones.

I'm partial to the fact that Legree was simply an evil twisted person but his reasoning is that he gets more out of them by working them until they die and then he just buys cheaper slaves.