A method is derived for estimating the lifetime of engineering polymers and composites acted upon by steady loads. The method applies a recently developed theory of crack growth in polymers. The basic properties that enter the analysis are the viscoelastic creep function of the material and the energy content of the crack‐generated surface layer. In addition, the characteristic (failure) dimension of the body must be specified, together with the load (stress) level acting upon the body and a stress‐distribution parameter. The final result of the derivation is a single, simple formula that expresses time to failure as a function of the input parameters. The method has been compared with experimental data upon arimid composite‐material strands, with encouraging results. The resulting predictions are fundamentally different from those of the Zhurkov formula, which is based upon an activation‐energy characterization for molecular schism. The model has been cast into a statistical form to accommodate a realistic treatment of data having wide scatter.