The Dyakonov-Perel mechanism, intimately related to the spin splitting of the electronic states, usually dominates the spin relaxation in zinc-blende semiconductor quantum structures. Previously it has been formulated for the two limiting cases of low and high temperatures. Here we extend the theory to give an accurate description of the intermediate regime which is often relevant for room temperature experiments. Employing the self-consistent multiband envelope function approach, we determine the spin splitting of electron subbands in n-(001) zinc-blende semiconductor quantum structures. Using these results we calculate spin relaxation rates as a function of temperature and obtain excellent agreement with experimental data.