Hot-electron cooling dynamics in photoexcited bulk and quantum-well GaAs structures were determined using time-correlated single-photon counting of photoluminescence (PL) decay. Hot-electron cooling curves were generated from analyses of the time-resolved PL spectra. The time constant characterizing the hot-electron energy-loss rate, avg, was then determined, taking into account electron degeneracy and the time dependence of the quasi-Fermi-level. This analysis was also applied to earlier data obtained by Pelouch et al. with the same samples, but based on PL up-conversion experiments with <80 fs temporal resolution. Both sets of experiments and analyses show that the hot-electron cooling rate can be much slower in GaAs quantum wells compared (at the same photogenerated carrier density) to bulk GaAs when this density is above a critical value. This critical density was found to range from high 1017 to low 1018 cm-3, depending upon the experimental technique; at the highest carrier densities, values of avg for quantum wells were found to be many hundreds of ps.