Question: How long was the concert career of the woman that is speculated to be the frustrated love inspiration of Brahms' Liebeslieder Walzer, Op. 52?

Context articles: List of Liebeslieder Walzer stagings
Neue Liebeslieder
Kenneth Lee Spencer
Liebeslieder Walzer (ballet)
Liebeslieder Walzer, Op. 52 (Brahms)
The Love Songs of Hafiz
Liebeslieder, Op. 114 (Strauss)
List of compositions by Sergei Rachmaninoff
Alto Rhapsody
Clara Schumann

This is a chronological list of stagings of New York City Ballet co-founder and balletmaster George Balanchine's Liebeslieder Walzer, made to the eponymous music by Brahms.
Neue Liebeslieder (New love songs), Op.
 65, written by Johannes Brahms, is a collection of Romantic pieces written for four solo voices and four hands on the piano.
 They are also known as "Neue Liebesliederwalzer".
 "Neue Liebeslieder" were written during the Romantic period between 1869 and 1874.
 The text of the songs is adapted from folk songs of various areas of Europe including Turkey, Poland, Latvia and Sicily.
 The text for songs 1 through 14 were translated and compiled by Georg Friedrich Daumer in his poem series, "Polydora"; the text for the fifteenth and final song, entitled "Zum Schluß" (In Conclusion), was written by Johann Wolfgang von Goethe.
Kenneth Lee Spencer (25 April 1913 - 25 February 1964), usually known as Kenneth Spencer, was an African-American opera singer and actor.
 A talented bass-baritone, Spencer starred in a few Broadway musicals and musical films in the United States during the 1940s.
 Frustrated with the racial prejudice he experienced in the United States, Spencer moved to West Germany in 1950 where he had a successful opera and concert career.
 He also appeared in a number of German films.
 His career was cut short when he died in the crash of Eastern Air Lines Flight 304.
Liebeslieder Walzer is a ballet by New York City Ballet co-founder and ballet master George Balanchine, based on the Brahms' "Liebeslieder Walzer", Op. 52, and their sequel "Neue Liebeslieder", Op. 65.
Johannes Brahms' Liebeslieder Waltzes ("Liebeslieder-Walzer") are distributed across two opus numbers: Op. 52 and Op.
 65.
 The waltzes are a collection of love songs in Ländler style for voices and piano four hands.
 The lyrics for the "Liebeslieder" come from Georg Friedrich Daumer's "Polydora", a collection of folk songs and love poems.
 While there is no concrete record indicating the exact inspiration for the Waltzes, there is speculation that Brahms' motivation for the songs was his frustrated love for pianist Clara Schumann, composer Robert Schumann's wife.
The Love Songs of Hafiz (German: "Des Hafis Liebeslieder)" is the name of two song cycles by Karol Szymanowski, Op. 24 with piano accompaniment, and Op. 26 with orchestral accompaniment.
 There are six songs in Op. 24, three of them orchestrated in Op. 26, and five additional songs, unique to Op. 26.
 They were composed in Vienna, Austria in 1911 and 1914, respectively.
 The works represent a transition between the first and second periods of the composer's style.
"Liebeslieder " op. 114 is a waltz by Johann Strauss II written in 1852 (not to be confused with Brahms's similarly titled "Liebeslieder Walzer" and "Neue Liebeslieder").
 At the time it was conceived, the waltz was titled 'Liebesgedichte' or "Love Poems" and during its first performance, it was even announced as 'Liebesständchen' or "Love Serenade".
 The first performance was at the famed Vienna Volksgarten on 18 June 1852 under the composer's direction.
The compositions of Sergei Rachmaninoff (1873–1943) cover a variety of musical forms and genres.
 Born in Russia, he studied at the Moscow Conservatory with Nikolai Zverev and Anton Arensky, and while there, composed some of his most famous works, including the first piano concerto (Op. 1) and the Prelude in C-sharp minor (Op. 3, No. 2).
 Although spread over three different opuses, he did go on to complete an important set of 24 preludes in all the major and minor keys.
 His Symphony No. 1 (Op. 13) was one of his first compositions as a "Free Artist" after graduation, and subsequently his first critical failure.
 The derision he received sent him into depression.
 After being sent through autosuggestive therapy, he composed his second piano concerto (Op. 18), which is still part of the major orchestra repertoire today.
 In 1909, he made his first tour of the United States, and composed Piano Concerto No. 3 (Op. 30), notable for its difficult cadenza.
 After this, due to migration from Russia in 1917 and his busy concert career, his output as a composer decreased, and during this period, he completed only six compositions.
 His last work, "Symphonic Dances" (Op. 45), was completed in 1940.
The Alto Rhapsody, Op.
 53, is a composition for contralto, male chorus, and orchestra by Johannes Brahms, a setting of verses from Johann Wolfgang von Goethe's "".
 It was written in 1869, as a wedding gift for Robert and Clara Schumann's daughter, Julie.
 Brahms scholars have long speculated that the composer may have had romantic feelings for Julie, which he may have integrated into the text and music of the "Alto Rhapsody".
 The text, with its metaphysical portrayal of a misanthropic soul who is urged to find spiritual sustenance and throw off the shackles of his suffering, has powerful parallels in Brahms's life and character.
Clara Schumann ("née" Clara Josephine Wieck; 13 September 1819 – 20 May 1896) was a German musician and composer, considered one of the most distinguished pianists of the Romantic era.
 She exerted her influence over a 61-year concert career, changing the format and repertoire of the piano recital and the tastes of the listening public.
 Her husband was the composer Robert Schumann.
 Together they encouraged Johannes Brahms.
 She was the first to perform publicly any work by Brahms.
 She later premiered some other pieces by Brahms, notably the "Variations and Fugue on a Theme by Handel".