Objective: This study aims to explore the time threshold for defining prolonged mechanical ventilation (PMV) in children, along with its risk factors and outcomes. Methods: A prospective cohort study was conducted on children aged 29 days-18 years, who were consecutively admitted to two Pediatric Intensive Care Units (PICUs) at Children's Hospital of Chongqing Medical University, from October 2020 to June 2021. The study included patients receiving mechanical ventilation (MV) for more than 2 days (each day >6 h). Participants were divided into five groups based on the duration of MV (2-7 days, 8-14 days, 15-21 days, 21-30 days, >30 days) to compare rates of extubation failure, all-cause mortality one month post-discharge, incidence of ventilator-associated pneumonia, tracheotomy rates, total hospital stay, PICU stay, and overall hospital costs. The most clinically and statistically significant outcome variables were selected. The Youden index was used to determine the MV duration with the most significant impact on overall outcomes, defining this as PMV. Baseline characteristics, treatment information, and outcomes were compared between PMV and non-PMV groups. Univariate and multivariate logistic regression analyses were used to identify risk factors for PMV occurrence. Results: A total of 382 subjects were included in the study. The distribution of children across the five MV duration groups was 44.2%, 27.7%, 10.7%, 8.9%, and 8.4% respectively. The rates of at least one extubation failure in each group were 5.9%, 10.4%, 41.5%, 41.2%, and 46.9% (p < .05). Statistically significant differences were observed among groups in terms of tracheotomy rates, all-cause mortality at 1 month postdischarge, median total hospital stay, median PICU stay, and hospital costs (p < .05). Defining PMV, the most appropriate time point calculated was 12.5 days, based on at least one extubation failure and/or death within 1 month postdischarge. Higher PIM-3 scores, weight for age <-2SD, admission for respiratory distress/insufficient ventilation and/or hemodynamic instability/shock/arrhythmia, noninvasive ventilation on the first day, and undergoing blood transfusion treatment were identified as risk factors for PMV (p < .05). Conclusion: In children, MV for >= 13 days significantly increases mortality rates, extubation failure and tracheotomy rates, duration of PICU and total hospital stay and costs. We suggest defining PMV as MV >= 13 days, particularly for children undergoing MV for respiratory illnesses. This definition can assist clinicians in developing appropriate treatment strategies by focusing on risk factors and providing reliable prognostic consultation to patients' families.