Background: How patient characteristics and outcomes vary according to the duration of heart failure (HF) is unknown in individuals with mildly reduced or preserved ejection fraction. We compared these, and the efficacy and safety of dapagliflozin, according to the time from diagnosis of HF in a prespecified analysis of the DELIVER trial (Dapagliflozin Evaluation to Improve the Lives of Patients With Preserved Ejection Fraction Heart Failure). Methods: HF duration was categorized as ≤6 months, >6 to 12 months, >1 to 2 years, >2 to 5 years, or >5 years. The primary outcome was the composite of worsening HF or cardiovascular death. The effect of treatment was examined by HF duration category. Results: The number of patients in each category was as follows: 1160 (≤6 months), 842 (>6 to 12 months), 995 (>1 to 2 years), 1569 (>2 to 5 years), and 1692 (>5 years). Patients with longer-duration HF were older and had more comorbidities with worse symptoms. The rate of the primary outcome (per 100 person-years) increased with HF duration: ≤6 months, 7.3 (95% CI, 6.3 to 8.4); >6 to 12 months, 7.1 (6.0 to 8.5); >1 to 2 years, 8.4 (7.2 to 9.7); >2 to 5 years, 8.9 (7.9 to 9.9); and >5 years, 10.6 (9.5 to 11.7). Similar trends were seen for other outcomes. The benefit of dapagliflozin was consistent across HF duration category: the hazard ratio for the primary outcome in the ≤6-month group was 0.67 (95% CI, 0.50 to 0.91); >6 to 12 months, 0.78 (0.55 to 1.12); >1 to 2 years, 0.81 (0.60 to 1.09); >2 to 5 years, 0.97 (0.77 to 1.22); and >5 years, 0.78 (0.64 to 0.96; P _interaction =0.41). The absolute benefit was greatest in longest-duration HF; the number needed to treat for HF >5 years was 24 versus 32 for ≤6 months. Conclusions: Patients with longer-duration HF were older, had more comorbidities and symptoms, and had higher rates of worsening HF and death. The benefits of dapagliflozin were consistent across HF duration. Even patients with long-standing HF and generally mild symptoms are not stable, and it is not too late for such patients to benefit from a sodium–glucose cotransporter 2 inhibitor. Registration: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT03619213.