Ideally, you should purchase liver from an animal that was slaughtered no more than 72 hours ago. The koshering process itself should be performed within 72 hours.
[header] How to kosher liver [title] Be particular about the liver you buy. [step] The liver should come from a kosher species (cow, calf, or chicken) that was slaughtered in the matter described by the torah. [substeps] The fats of the liver should have been removed during the time of slaughter.