Electrostatic breakdown leads to the majority of anomalies and failures attributed to spacecraft interactions with the plasma space environment. It is therefore critical to understand how electrostatic field strength (FESD) of spacecraft materials varies due to environmental conditions such as duration of applied electric field, rate of field change, history of exposure to high fields, and temperature. We have developed a dual-defect, thermodynamic, mean-field trapping model in terms of recoverable and irrecoverable defect modes to predict probabilities of breakdown. Fits to a variety of measurements of the dependence of FESD of insulating polymers on endurance time, voltage ramp rate, and temperature based on this model yield consistent results. Our experimental results for the prototypical materials low density polyethylene (LDPE) and polyimide (PI or Kapton HN) suggest that values of FESD from standard handbooks, or cursory measurements that have been used routinely in the past, substantially overestimate the field required for breakdown in common spacecraft applications, which often apply sub-critical fields for very long time periods as charge accumulates.