  We revisit the popular notion of effective or renormalized charge, which is a
concept of central importance in the field of highly charged colloidal or
polyelectrolyte solutions. Working at the level of a linear Debye-H\"uckel like
theory only, we propose a simple, efficient and versatile method to predict the
saturated amount of charge renormalization, which is however a non-linear
effect arising at strong electrostatic coupling. The results are successfully
tested against the numerical solutions of Poisson-Boltzmann theory for polyions
of various shapes (planar, cylindrical and spherical), both in the infinite
dilution limit or in confined geometry, with or without added electrolyte. Our
approach, accurate for monovalent micro-ions in solvents like water, is finally
confronted against experimental results, namely the crystallization of charged
colloidal suspensions and the osmotic coefficient of B-DNA solutions.
