It's been 50 years since Quantum Chromodynamics (QCD), or its asymptotic freedom, was found. Yet, the quantitative understanding of
quark-hadron duality, an underlying assumption in the connection between perturbative QCD and experiments, is lacking. The duality implies that the QCD processes summed over all final hadronic states can be calculated with quarks (and gluons) as fundamental degrees of freedom. But, what is the associated systematic uncertainty? That should be able to be answered using lattice QCD, which allows us to perform fully non- perturbative computation. We discuss how this can be done taking a few examples including the inclusive semi-leptonic B meson decays.