Question: Wilhelmine Schröder was the royal mistress of a man that was King of Sweden beginning in what year?

Context paragraphs:
Die schöne Wilhelmine: Die schöne Wilhelmine ("the beautiful Wilhelmine") is a 1965 novel by the German writer Ernst von Salomon.  It tells the story of the romance between the Prussian king Friedrich Wilhelm II and his mistress Wilhelmine Encke during the second half of the 18th century.

Mariana Koskull: Henrietta Mariana "Marianne" Charlotta Koskull (1785–1841) was a Swedish noble and lady-in-waiting, known as the royal mistress of King Charles XIII of Sweden and King Charles XIV John of Sweden.

Wilhelmine von Grävenitz: Wilhelmine von Grävenitz (1684–1744) was a German noblewoman, the favorite and royal mistress of Duke Eberhard Ludwig of Württemberg between 1706 and 1731.

Hedvig Taube: Hedvig Ulrika Taube (31 October 1714 – 11 February 1744), also Countess von Hessenstein was a Swedish courtier and countess, a Holy Roman countess of the Empire, and royal mistress to king Frederick I of Sweden from 1731 to 1744.  She is regarded as the only official royal mistress in Swedish history.

Wilhelmine Schröder-Devrient: Wilhelmine Schröder-Devrient, born Wilhelmine Schröder (6 December 180426 January 1860), was a German operatic soprano.  As a singer, she combined a rare quality of tone with dramatic intensity of expression, which was as remarkable on the concert platform as in opera.  Today, she is remembered as the author of the most celebrated erotic novel in the German language.

Catharina Ebba Horn: Catharina Ebba Horn af Åminne, (27 May 1720 – 12 September 1781 in Jakobsberg), was a Swedish noble, a German Roman Countess and the second official royal mistress of king Frederick I of Sweden from 1745 to 1748.  She was one of two official royal mistresses in Sweden.  She had no influence over state affairs, but she did patronise careers through her powerful connections and her position.

Wilhelmine Dorothee von der Marwitz: Wilhelmine Dorothee von der Marwitz (April 1718-16 January 1787), was the royal mistress of Frederick, Margrave of Brandenburg-Bayreuth, from the late 1730s until 1744.  She was also the host of an influential salon in Vienna in the 1780s.

Stella de’ Tolomei: Stella de’ Tolomei (d. 1419), was an Italian courtier and royal mistress.  She was the official royal mistress of Niccolò III d'Este, Marquis of Ferrara for many years and the mother of several of his acknowledged illegitimate children.

Charles XV of Sweden: Charles XV & IV also Carl ("Carl Ludvig Eugen"); Swedish: "Karl XV" and Norwegian: "Karl IV" (3 May 1826 – 18 September 1872) was King of Sweden ("Charles XV") and Norway ("Charles IV") from 1859 until his death.  Though known as King Charles XV in Sweden (and also on contemporary Norwegian coins), he was actually the ninth Swedish king by that name, as his predecessor Charles IX (reigned 1604–1611) had adopted a numeral according to a fictitious history of Sweden.

Wilhelmine Schröder: Fredrika Vilhelmina "Wilhelmine" Augusta Schröder (23 September 1839 - died 13 May 1924), was a Swedish telegraphist, writer and journalist, and was also the confidant and royal mistress of King Charles XV of Sweden.

Supporting paragraphs: Wilhelmine Schröder Charles XV of Sweden
Answer: 1859