"How Long" is a 1974 song by the British group Ace from their album Five-A-Side. It reached No. 3 in the US and Canadian charts, and No. 20 in the UK Singles Chart.

Although widely interpreted as being about adultery, the song was in fact composed by lead singer Paul Carrack upon discovering that bassist Terry Comer had been secretly working with the Sutherland Brothers and Quiver. Comer returned to Ace in time to play on the song.

The guitar solo is by Alan "Bam" King.

In 1976, Bobby Womack recorded a version that appeared on the Home Is Where the Heart Is album.

In 1977, Barbara Mandrell recorded a country/disco version of the song on her Love's Ups and Downs album.

In 1981, Rod Stewart covered the song on the album Tonight I'm Yours. It charted in the top fifty of the Billboard Hot 100.

In 1981, Lipps Inc. released a cover of this song which reached No. 4 on the U.S. dance chart, No. 29 on the U.S. soul singles chart, and No. 42 in Canada.

In 1989, Barbara Dickson released the song as part of the Coming Alive Again album.

In 1993, British reggae group Aswad and singer Yazz covered the song which reached No. 31 in the UK Singles Chart, from Yazz's 1994 album One on One.

In 1996, Paul Carrack included his own version of the song on his album Blue Views.

In 2013, it was covered by smooth jazz artists Jeff Golub and Brian Auger, on their Train Keeps A Rolling album, with vocals by Christopher Cross.

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