Background: In 2016, the International Study Group of Pancreatic Fistula (ISGPF) improved the definition of postoperative pancreatic fistula (POPF) which defines one of diagnostic criteria for progression of biochemical leak (BL) to grade B fistula as drain for more than 3 weeks. However, we found patients with BL after pancreaticoduodenectomy (PD) or laparoscopic pancreatoduodenectomy (LPD) in our medical center usually progressed to grade B fistula within 3 weeks, which is inconsistent with diagnostic criteria (more than 3 weeks) described in the guidelines. The objective of this study was to investigate the time point at which patients with BL progressed to grade B fistula after PD or LPD. Methods: We selected patients from June 2013 to October 2022 as the research subjects. We used a receiver operating characteristic (ROC) curve to calculate the cutoff value of the time point when BL progressed to grade B fistula. Results: The cutoff value of the time point at which BL progresses to grade B fistula was postoperative day (POD) 9.5. The sensitivity and specificity were 63.6% and 81.4%, respectively. Conclusion: The cutoff value of time point of BL progression to grade B fistula was 9.5, which indicated that we do not need to define grade B fistula as drain for more than 3 weeks. We considered that a too-late time point might delay the timely diagnosis and treatment of grade B fistula and further optimization of the definition of grade B fistula was necessary.