The time formative time lag (t f ) in the breakdown process of polymers on a nanosecond time scale is calculated. By reviewing the mechanisms related to breakdown of solid dielectrics, it is concluded that the avalanche breakdown mechanism is mainly responsible for the breakdown phenomenon of polymers on a nanosecond time scale, which is similar to the Townsend theory of gas breakdown under short pulses. In addition, the maximum functioning time of this theory is ascertained as 1 μs. When the time is longer than 1 μs, the thermal breakdown mechanism and other mechanisms begin to play the role. A formula to calculate t f for the avalanche breakdown mechanism is presented. This formula takes into account different factors that have an influence on t f , such as the dielectric size, type and purity. With this formula, it is calculated that t f for common polymers with a thickness ranging from 0.001-1 mm is about 0.01-100 ns. Experimental results in literatures were summarized to support the calculation results. The formative time lag is a key parameter to evaluate the pulse width effect on the electric breakdown strength on a nanosecond time scale.