Songfacts®:

• The follow-up to Charlie Puth's hit song "Attention," this finds him apologizing to a lover for being unfaithful.
  
  The song's chorus is from the girl's perspective, in which she searches for details about his affair.
  
  How long has this been goin' on?
  You've been creepin' 'round on me
  While you're callin' me "baby"
  How long has this been goin' on?
  
  Charlie admitted in an interview with Access Hollywood that the brutally honest lyrics are about something he has experienced in life.
• The track opens with a slinky bassline. Speaking with Ryan Seacrest, Puth explained:
  
  "It was originally supposed to be in E-flat minor. With the theme of 'Attention' kinda being a 'bass' song, I didn't want it to be, like, a ripoff of 'Attention,' but lyrically, the song is like part two of 'Attention,' as far as the story goes. I'll leave that up to interpretation. In my mind, it had to start out with a bass, but not so much a bass with so much melody, just like a [makes noise], like a walking bassline."
• The song's music video was shot in downtown Los Angeles by Emil Nava. It finds Puth dancing on the sidewalk and in an empty office building, as well as walking on walls and even flying. Asked by MTV News why he wanted to show some of his moves in the clip, he replied:
  
  "Because I can't dance! I'm very self-deprecating; I've been making fun of myself since the day I was born. If I did one more video where I was making out with a girl, I was going to throw up. I was so sick of that, so I decided to do something completely different. The song, the lyrics - it's kind of part two of "Attention," where there's a somber tone to it but a happy beat. And I don't really know why I'm dancing to this song. I don't know why I'm flying in the air. I just wanted to do something random. There's really no reason. Everyone's going to come up with all these theories, like, 'Oh, he's dancing to celebrate that he's free.' No. I just wanted to dance like an idiot."
• Charlie Puth produced the track himself and co-wrote it with Jacob Kasher (Cobra Starship's "Good Girls Go Bad," Maroon 5's "Sugar")."
  
  "He brought 'How Long' to me with the melodies and beat pretty much done the way you hear it on the radio," Kasher recalled to Billboard. "Charlie explained to me his vision of the song and said, 'Do you like this? Do you want to write it?'"
• The song was inspired by the classic blue-eyed soul sounds of Hall & Oates. "It was summer," Puth recalled to Billboard. "I was just about to go on tour with my buddy Shawn Mendes, and I put on Hall & Oates…and I was like, 'Okay, I wanna make a record like that.'"
  
  Puth added that called up one of his A&R pals and said, "Wouldn't it be cool, like if there was a song called 'How Long Has This Been Going On?' and each kick drum would be on the lyric?"
  
  "I had one broken Rokit speaker to produce this record on," he continued. "Every night after I would get off stage, I'd just kind of build the track out even more and I still wanted it to be like, thematically with a bass. My bass player Dmitry came in and he had this sick preamp and he plugged it in and it sounded bouncy … like me walking with a big cup of coffee down the street. Like Fat Albert or Mickey Mouse when he drops the piano."
• The song shares the same title and "How long has this been going on?" part of the refrain with Ace's 1974 hit song "How Long?."