Piping erosion is one of the main cause of serious hydraulic work failure. This phenomenon involves the progression of a continuous tunnel between the upstream and the downstream side. Starting from the bulk equations for diphasic flow with diffusion and from the jump equations with erosion, we propose a two parameters scaling law for piping erosion. The first parameter is the critical shear stress. The second parameter is the erosional characteristic time, which is a function of the initial hydraulic gradient and of the erosion coefficient. The comparison with experimental data has confirmed the validity of the scaling law. We deduce an estimation of the time for breaching in hydraulic works (dams, dykes).