Light scattering has been used to investigate the effect of shear-induced demixing (SID) in polystyrene/poly(vinyl methyl ether) blends of diverse molecular weight distributions down to -30 K below the quiescent spinodal temperature of any blend. The early stage of the demixing process, observed along the vorticity direction, presents features remarkably similar to spinodal decomposition. These include an initial exponential increase of scattered intensity with time and a maximal growth rate for q = q(m). This demixing occurs after a shearing time tau(d), which decreases as shear rate increases. The shear-induced demixing process appears to be strongly dependent on the applied shear rate, the experimental temperature, and the polydispersity of the components of the blend. For a given shear rate, this process is faster (higher growth rates and lower tau(d)) for blends with a higher polydispersity and for lower experimental temperatures. These results allow quantification of the SID process and give access to data which can test the theories predicting SID by taking into account the effect of stored energy on the thermodynamics or the influence of shear flow on concentration fluctuations.