BackgroundPostoperative adjuvant chemotherapy (AC) is now well-accepted as standard for high-risk stage II and stage III colorectal cancer (CRC) patients, however the optimal time to initiate AC remains elusive.MethodsA comprehensive literature search was performed using the PubMed and Embase databases. The Hazard ratio (HR) with the corresponding 95% confidence interval (CI) was used as an effect measure to evaluate primary endpoints. All analyses were conducted using Stata software version 12.0 with the Random-effects model.ResultsA total of 30 studies were included in our study. Upon comparison on overall survival (OS), we identified that delaying the initiation of AC for > 8 weeks after operation was significantly associated with poor OS (HR: 1.37; 95% CI: 1.27—1.48;P< 0.01). The poor prognostic value of AC delay for > 8 weeks was not undermined by subgroup analysis based on region, tumor site, sample size and study quality. No obvious differences were observed in survival between AC within 5–8 weeks and ≤ 4 weeks (HR: 1.03; 95% CI: 0.96 -1.10;P= 0.46). Moreover, two studies both highlighted that the survival benefit of AC was still statistically significant when AC was applied 5–6 months after surgery compared with the non-chemotherapy group.ConclusionsDelaying the initiation of AC for > 8 weeks after surgery was significantly associated with poor OS. AC started within 8 weeks after surgery brought more benefits to CRC patients. There were no obvious differences in survival benefits between AC within 5–8 weeks and ≤ 4 weeks. Compared to patients not receiving AC after surgery, a delay of approximately 5–6 months was still useful to improve prognosis.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12885-023-10863-w.