The Black Hills Lands Claim is an ongoing land dispute between the US Government and The Sioux Nation. The treaty of Fort Laramie protected the hills from white settlement, but the discovery of gold on the land led to its defacto seizure in 1874, and officially by a congressional Act in 1877. The legal struggle for the return of the lands started in
the early 1920s under tribal lawyer Richard Case where he argued that the 1877 Act of February was illegal and that the United States never made a legitimate purchase of the land. Tribal Lawyers Marvin Sonosky and Arthur Lazarus took over the case in 1956 until they won in 1980 when the case reached the supreme court (United States vs The Sioux Nation of Indians) which upheld an earlier ruling that the 1877 Act was in violation of the 5th amendment.
However in a unanimous decision the Sioux Tribal Council refused to accept the awarded Compensation with the intention of forcing the US Government to return the land.
Thus the legal struggle is roughly 90 years; is this the longest legal case in the US or elsewhere, or are there others go beyond this one?