CREATE TABLE table_37104 ("Name" CLOB, "Birth" CLOB, "Marriage" CLOB, "Became Duchess" CLOB, "Ceased to be Duchess" CLOB, "Death" CLOB, "Spouse" CLOB)

When did the duchess who became duchess on 7/8 April 1766 cease to be duchess?
SELECT "Ceased to be Duchess" FROM table_37104 WHERE "Became Duchess" = '7/8 april 1766'
SELECT "Ceased to be Duchess" FROM "table_37104" WHERE "Became Duchess" = '7/8 april 1766'