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

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'