"How Long" is a song recorded and produced by American singer Charlie Puth. He co-wrote the song with Jacob Kasher and DJ Frank E. It was released on October 5, 2017, by Atlantic Records as the second single from Puth's upcoming second studio album, Voicenotes (2018). Commercially, it has reached the top 10 in Israel, Malaysia, the Philippines, Scotland, and the United Kingdom as well as the top twenty in Australia, Belgium, Denmark, Ireland, the Netherlands, New Zealand, Slovakia, Spain, and Switzerland.

Puth teased the song in September 2017 on social media. He tweeted two photos in a row, which were captioned "how" and "long" respectively. In October 2017, he posted photos and video clips with the captions, "has", "this", "been" and "going". On October 4, 2017, he posted a video of the voice note recording behind the song with the caption "on", eventually spelling out "how long has this been going on" altogether, as one of the lines in the song's lyrics.

The song is composed in the key of C♯ minor with a tempo of 108-112 beats per minute.

Chris Malone of Billboard felt the song "opens with a slinky bassline 'Attention'", and features a catchy chorus. He thinks that the song "marks a slight stylistic shift from the more straight forward pop sound that filled his debut album Nine Track Mind", and that Puth took "the funkier R&B-leaning direction" with "How Long". James Dinh of iHeartRadio deemed it "a fitting follow-up to 'Attention' as the 25-year-old tackles yet another vulnerable narrative from the romance department", and expects it to enter top 10 on Billboard Hot 100. Mike Wass of Idolator wrote that the song "picks up where 'Attention' left off". He described the song as "instantly addictive and could well turn out to be an even bigger hit". Katrina Rees of CelebMix regarded the song as "Charlie's latest infectious offering", and wrote that it "has a slick yet fairly stripped back vibe which showcases the singer's vocal range".

"How Long" reached the top 10 on seven charts, including the UK Singles Chart. It debuted at number 60 on the Billboard Hot 100 on the week of October 28, 2017. It has so far peaked at number 26.

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