Applying four-wave mixing spectroscopy to a high-quality GaN/AlGaN single quantum well, we report on the experimental determination of excitonic dephasing times at different temperatures and exciton densities in IIInitride heterostructures. By comparing the evolutionwith the temperature of the dephasing and the spin-relaxation rate, we conclude that both processes are related to the rate of excitonic collisions. When spin relaxation occurs in the motional-narrowing regime, it remains constant over a large temperature range as the spin-precession frequency increases linearly with temperature, hence compensating for the observed decrease in the dephasing time. From those measurements, a value of the electron-hole exchange interaction strength of 0.45 meV at T = 10K is inferred.