Photoluminescence spectra of nonpolar M-plane ZnO/Zn0.8Mg0.2O quantum wells exhibit strong excitonic peaks from low (10 K) to high (325 K) temperatures. We find that the total integrated intensity remains quasiconstant and that the exciton lifetime increases linearly with the temperature from a value of 750 ps at 100 K until about 2.4 ns at 325 K. This behavior is well described by an original model accounting for the exciton phase space filling. This indicates that radiative recombination of free excitons is dominating the quantum well photoluminescence even at room temperature.