  The evolution and lifetimes of thermally pulsating asymptotic giant branch
(TP-AGB) stars suffer from significant uncertainties. We present a detailed
framework for constraining model luminosity functions of TP-AGB stars using
resolved stellar populations. We show an example of this method that compares
various TP-AGB mass-loss prescriptions that differ in their treatments of mass
loss before the onset of dust-driven winds (pre-dust). We find that models with
more efficient pre-dust driven mass loss produce results consistent with
observations, as opposed to more canonical mass-loss models. Efficient pre-dust
driven mass-loss predicts for [Fe/H] < -1.2, lower mass TP-AGB stars (M < 1
Msun) must have lifetimes less than about 1.2 Myr.
