Question: Since when has the husband of Diana, Princess of Wales been heir apparent?

Context paragraphs:
Crown Prince of Thailand: The Crown Prince of Thailand (or Siam; Thai: สยามมกุฎราชกุมาร ; rtgs: Sayammakutratchakuman ) is a title held by the heir apparent to the Thai throne.  First created by King Chulalongkorn (Rama V) in 1886, for his son Prince Maha Vajirunhis, the king's eldest son by a royal wife Queen Savang Vadhana.  Prior to this, the Siamese throne did not have a law or formal system regulating the royal succession.  In 1688 King Petracha of Ayutthaya created the title of Front Palace, which by the Rattanakosin period had become the main title granted to the heir presumptive to the throne.  However few Front Palaces have succeeded to the throne this way, with the exception of King Buddha Loetla Nabhalai (Rama II) in 1809.  After the Front Palace crisis in 1875, the title of Front Palace was abolished and replaced with the title of Crown Prince, who became heir apparent to the throne.

Diana: In Her Own Words: Diana: In Her Own Words is a television documentary that was broadcast on Channel 4 in the United Kingdom on 6 August 2017.  The film includes footage of the late Diana, Princess of Wales discussing aspects of her personal life, including her marriage to Prince Charles which was recorded during conversations she had with a voice coach in 1992 and 1993.

List of heirs to the Prussian throne: This is a list of those people who were heir apparent or heir presumptive to the Kingdom of Prussia from its foundation in 1701 to the end of the monarchy in 1918.  From 18 January 1871 the Crown Prince of Prussia was also heir apparent to the German Empire.  Those heirs who succeeded are shown in bold.

Diana, Princess of Wales: Diana, Princess of Wales (Diana Frances; "née" Spencer; 1 July 1961 – 31 August 1997), was a member of the British royal family as the first wife of Charles, Prince of Wales, who is the eldest child and heir apparent of Queen Elizabeth II.

Qian Hongzun: Qian Hongzun (錢弘僔) (925－June 7, 940), formally Heir Apparent Xiaoxian (孝獻世子, "the filial and wise heir apparent"), was an heir apparent to the throne of the Chinese Five Dynasties and Ten Kingdoms Period state Wuyue during most of the reign of his father Qian Yuanguan (né Qian Chuanguan, King Wenmu), but did not inherit the throne on account of his predeceasing his father.

Duke of Rothesay: Duke of Rothesay (Scottish Gaelic: "Diùc Baile Bhòid" , Scots: "Duik o Rothesay" ) is a dynastic title of the heir apparent to the British throne, currently Prince Charles.  It was a title of the heir apparent to the throne of the Kingdom of Scotland before 1707, of the Kingdom of Great Britain from 1707 to 1801, and now of the United Kingdom of Great Britain and Northern Ireland.  It is the title mandated for use by the heir apparent when in Scotland, in preference to the titles "Duke of Cornwall" (which also belongs to the eldest living son of the monarch, when and only when he is also heir apparent, by right) and "Prince of Wales" (traditionally granted to the heir apparent), which are used in the rest of the United Kingdom and overseas.  The Duke of Rothesay also holds other Scottish titles, including those of Earl of Carrick, Baron of Renfrew, Lord of the Isles and Prince and Great Steward of Scotland.  The title is named after Rothesay on the Isle of Bute, Argyll and Bute, but is not associated with any legal entity or landed property, unlike the Duchy of Cornwall.

Sukhrangpha: Sukhrangpha was the king of Ahom kingdom from 1332 CE to 1364 CE.  He had to face the revolt led by his youngest brother Chao Pulai or Tai Sulai (there are confusion in Ahom historians whether Chao Pulai and Tai Sulai were same or different personality.  Eventually he came in terms with Chao Pulai (or Tai Sulai) by appointing him as Charing Raja, a newly created official post to administer the region of Charing.  Later the post of Charing Raja was usually conferred to the heir apparent to the throne.  It can be compared with the title of Prince of Wales of England, where the heir apparent to the throne was conferred with this title.

Charles, Prince of Wales: Charles, Prince of Wales (Charles Philip Arthur George; born 14 November 1948) is the eldest child and heir apparent of Queen Elizabeth II.  Known alternatively in Cornwall as Duke of Cornwall and in Scotland as Duke of Rothesay, he is the longest-serving heir apparent in British history, having held the position since 1952.  He is also the oldest person to be next in line to the throne since Sophia of Hanover (the heir presumptive to Queen Anne), who died in 1714 at the age of 83.

Dauphine of France: The Dauphine of France (] ) was the wife of the Dauphin of France (the heir apparent to the French throne).  The position was analogous to the Princess of Wales (the wife of the heir apparent to the British throne).

Prince of Beira: Prince of Beira (Portuguese: "Príncipe da Beira" ) is a title traditionally granted to the heir apparent of the heir apparent to the throne of Portugal.  The title's original use that it be granted on the eldest daughter of the reigning monarch of Portugal.  Tied with the title of Prince of Beira, is Duke of Barcelos, as heir to the Duke of Braganza and Prince of Brazil ("later" Prince Royal of Portugal).  The current Prince of Beira is Prince Afonso, the eldest son of Duarte Pio, Duke of Braganza.

Supporting paragraphs: Diana: In Her Own Words Charles, Prince of Wales
Answer: 1952