The broadband impedance spectrum (BIS) has emerged as a promising technique for detecting minor defects in cables. Although minor defects may span a considerable length, existing research often overlooks the impact of defect length. This paper investigates the influence of insulation degradation defect length on the BIS and the equivalent cable length under two representative scenarios: communication cables and power cables. By analyzing the defective BIS with varying defect lengths and start positions, the relationship between defect length and equivalent cable length is examined. The results show that increasing the defect length leads to a linear rise in the equivalent cable length, with little sensitivity to defect start position. These findings provide valuable insights for BIS-based defect localization methods that account for defect length.