It is admitted that for 1kgCOD you can expect 0.35Nm3 of methane. But it is if you considered that all of the COD is biodegradable.
You can find a good discussion on this topic on this link: https://www.researchgate.net/post/What_is_the_relation_between_COD_of_Influent_waste_water_and_expected_biogas_production