  In the large $N_c$ limit, the nonleptonic width of the B meson is determined
by the lepton mass spectrum in semileptonic B decay and the hadron mass
spectrum in $\tau$ decay. This result can be compared with the nonleptonic
width computed using the operator product expansion and local duality, to
estimate the violation of local duality in B decay. In the absence of the
required data, we use experimental Monte Carlo predictions and find that the
difference is small, at the few percent level. We estimate the theoretical
uncertainty in our results from higher order corrections (including $1/N_c$
effects). We also identify a new contribution to the $\Lambda_b-B$ lifetime
difference.
