We analyze the magnitude of degenerate two-photon gain of several quantum wells (QWs) in a resonant microcavity which is pumped off-resonantly by an optical field. A set of coupled phenomenological rate equations for electron-hole pairs and photons is derived. In the framework of this model, we make a realistic prediction of the time-integrated two-photon gain and find that a lambda-cavity containing 11 QWs would give a maximal amplification of similar to 2.3% for a peak,intensity of similar to 80 MW/cm(2). A better insight into the dependence of the gain on. the various physical parameters is given by an analytical formula resulting from various approximations on the rate equations.