A model for the energy gain of inertial confinement fusion (ICF) targets is derived from realistic ignition profiles. The model considers the ablative implosion of a spherical shell driven by a rather general power pulse and takes into account the main physical limitations constraining the target performance. As a result, the model gives an interpretation of the gain curves obtained by numerical simulations and recently reported in the literature. The resulting scaling laws for the ignition energy and for the limiting gain are in good agreement with these simulations. Besides, several physical features associated with the target gain and observed in the simulations are well described. In particular, it is found that, along the limiting gain, the ICF parameter of the fuel is almost constant and, for applications to indirect drive fusion, the implosion velocity is proportional to the radiation temperature. The explicit scaling laws provided by the model allow a parametric study of the target gain as a function of the constraints on symmetry, stability and other physical quantities