For a wide class of nonlinear parabolic equations of the form u y ⌬ u s t Ž . F u, ٌu , we prove the nonexistence of global solutions for large initial data. We also estimate the maximal existence time. To do so we use a method of comparison with suitable blowing up self-similar subsolutions. As a consequence, we improve several known results on u y ⌬ u s u p , on generalized Burgers' equations, and on t other semilinear equations. This method can also apply to degenerate equations of porous medium type and provides a unified treatment for a large class of problems, both semilinear and quasilinear.