In this article, a time constant of lateral charge loss (LCL) mechanism inside 3-D NAND flash memories during their data retention process is comprehensively investigated. 1-D multiscale simulation is conducted to investigate the lateral migration of charges, and Poole–Frenkel emission of charges has turned out to be the most significant factor that determines the time constant of long-term retention. Time constant is modeled as a function of threshold voltage change and trap energy level, and the suggested model makes a good quality fit with the technology computer-aided design (TCAD) simulation results. The results will be a great help to reduce computational and experimental costs that originate from TCAD simulations and needless experiments.