Elimination of Polish elites and intelligentia was part of Generalplan Ost. The Intelligenzaktion, a plan to eliminate the Polish intelligentsia, Poland's 'leadership class', took place soon after the German invasion of Poland, lasting from fall of 1939 till spring of 1940. As the result of this operation in 10 regional actions about 60,000 Polish nobles, teachers, social workers, priests, judges and political activists were killed. It was continued in May 1940 when Germany launched AB-Aktion, More than 16,000 members of the intelligentsia were murdered in Operation Tannenberg alone.
How long did liquidation occur?
fall of 1939 till spring of 1940