Question: How long has Princess Margaretha of Liechtenstein's brother reigned as Grand Duke?

Context paragraphs:
Princess Marie Gabriele of Luxembourg: Princess Marie Gabriele of Luxembourg, Countess of Holstein-Ledreborg (given names: "Marie Gabrielle Aldegunde Wilhelmine Louise"; born 2 August 1925) is the third daughter and fourth of the six children of Grand Duchess Charlotte of Luxembourg and her husband Prince Félix.  She is the younger sister of former Grand Duke Jean of Luxembourg and thus a paternal aunt to the current Grand Duke, Jean's son Grand Duke Henri.  In 1951 she married Danish Count Karl Johan Ludvig, 7th Count of Holstein-Ledreborg (2 October 1919 – 25 June 2001), Roman Catholic; the couple were married for 50 years until his death in 2001.  Together, they had seven daughters.

Henri, Grand Duke of Luxembourg: Henri (Luxembourgish: "Henri Albert Gabriel Félix Marie Guillaume" , ] ; born 16 April 1955) is the current Grand Duke of Luxembourg, reigning since 7 October 2000.  He is the eldest son of Grand Duke Jean and Princess Joséphine-Charlotte of Belgium, and a first cousin of Philippe, the current king of Belgium.

Princess Margaretha of Liechtenstein: Princess Margaretha of Liechtenstein (born "Princess Margaretha of Luxembourg" on 15 May 1957) is the fourth child and second daughter of Grand Duke Jean of Luxembourg and Princess Joséphine Charlotte of Belgium.  As the sister of Grand Duke Henri of Luxembourg and the sister-in-law of Prince Hans-Adam II of Liechtenstein, she is a princess of two current realms and a member of the Luxembourg and Liechtenstein reigning dynasties.  The Princess holds similar precedence in both monarchies, as sister and sister-in-law of the Sovereign.

Prince Carl Bernadotte: Carl Gustaf Oscar Fredrik Christian, Prince Bernadotte (10 January 1911 – 27 June 2003), originally Prince Carl, Duke of Östergötland, was the youngest child and only son of Prince Carl of Sweden and Princess Ingeborg of Denmark and eventually a prince of the Belgian nobility.  To distinguish himself from his father, he was widely known as "Carl Junior."  He was the brother of Princess Margaretha of Sweden, Queen Astrid of Belgium and Crown Princess Märtha of Norway.

Princess Amalia of Nassau: Princess Amalia of Nassau ("Amalia Gabriela Maria Teresa"; born 15 June 2014) is a Luxembourgish princess and the eldest child of Prince Félix and Princess Claire of Luxembourg.  She's the first and only granddaughter and third grandchild of Grand Duke Henri.  She is currently third in the line of succession, behind her paternal uncle Hereditary Grand Duke Guillaume and her father.  She has a brother, Prince Liam of Nassau.

Princess Margaretha of Saxony: Princess Margaretha of Saxony, Duchess of Saxony (German: "Prinzessin Margaretha Karoline Friederike Cecilie Auguste Amalie Josephine Elisabeth Maria Johanna von Sachsen, Herzogin zu Sachsen" ) (24 May 1840 – 15 September 1858) was the eighth child and fifth eldest daughter of John of Saxony and his wife Amalie Auguste of Bavaria and a younger sister of Albert of Saxony and George of Saxony.  She was born in Dresden, then in the Kingdom of Saxony.  Through her marriage to Archduke Charles Louis of Austria, Margaretha was a member of the House of Habsburg-Lorraine and an Archduchess and Princess of Austria and Princess of Hungary, Croatia, Bohemia, and Tuscany.

Princess Margaretha, Mrs. Ambler: Princess Margaretha, Mrs. Ambler, (Margaretha Désirée Victoria; born 31 October 1934) is a Swedish princess, the eldest sister of King Carl XVI Gustaf of Sweden and also first cousin of Queen Margrethe II of Denmark.

John Ambler: John Kenneth Ambler (6 June 1924 – 31 May 2008) was a British businessman married to Princess Margaretha of Sweden.

Prince Jean of Luxembourg: Prince Jean of Luxembourg (given names: "Jean Félix Marie Guillaume") was born on 15 May 1957 at Betzdorf Castle, the second son of Jean, Grand Duke of Luxembourg and Princess Joséphine-Charlotte of Belgium.  He is the twin brother of Princess Margaretha.  He frequently goes by the name of Jean Nassau.

Princess Margaretha of Sweden: Princess Margaretha of Sweden (Margaretha Sofia Lovisa Ingeborg; 25 June 1899 – 4 January 1977) was a member of the Swedish Royal Family and a Princess of Denmark by marriage, the eldest sister of Märtha, Crown Princess of Norway, Queen Astrid of the Belgians and Prince Carl Bernadotte.

Supporting paragraphs: Princess Margaretha of Liechtenstein Henri, Grand Duke of Luxembourg
Answer: since 7 October 2000.