Why not the other durations: A one- or two-year validity would demand re-certification too frequently for practical farm conversion cycles, while a four-year validity is longer than what current organic certification norms allow.
Under India's National Programme for Organic Production, once a farm completes conversion and is certified, the certificate is issued for a fixed cycle, with annual surveillance inspections carried out in between to confirm continued compliance.
That fixed validity period is three years, after which the farm must undergo recertification.