@mediaresearch advises that:
The average length of a Song in 2020 is 3 minutes and 42 seconds, down from 4 minutes and 22 seconds.
Introductory Movements have decreased from 13.1 seconds to 7.4 seconds.
Hip Hop accounts for 6 of the top ten on Billboard.
The average number of Songwriters per Song is four.
"The times they are a'changin'."

"Enough", "Not Enough", "Too Much".

An Introductory Movement only needs to be long enough to serve that function. Its function is to 'hook' listener interest long enough for the main body of the Composition to begin, a Verse or Chorus, if it is a vocalized Lyric, or just the 'new' sounds of whatever you give them next. A short Introductory Movement leaves more time for everything that follows. I call that 'hook' element 'Hook Factor'. All parts of the Composition have, or should have, Hook Factor.
This principle of a short Intro is part of the 'rule' "Don't bore us! Get to the Chorus!"
Whatever follows the Intro, the Chorus, if your Compositions opens with the Chorus, or Verse I, if that's the next component, should have Hook Factor. That first Line of Lyric, or that first instrumental sound, individual Notes in a Melody, or other 'sounds', should capitalize on the Hook Factor of the Intro, and sustain Hook Factor, keeping them interested. How long can you do that? If you keep the Composition moving, instrumentally or with the Lyrical Storyline, you can keep them all the way through the 3 minute 42 second time span. The Human attention span can be longer than that if you have a longer Composition, but for 'pop', meaning 'popular' music, in any genre, staying short can be a virtue. Start, entertain, and end.
A Composition must have 'Enough' Repetition and 'Enough' Change.
Enough Repetition supplies Structure the listener can relate to. They recognize the Verse Movement, the Change to the Chorus, a more emotional intensity, usually a higher pitch, and then the Repetition of the Melody they 'learned' in hearing Verse I when you employ it again in Verse II. I contend that even in instrumental or Rap/Hip Hop or Jazz genres that adherence to Repetitional/Change Structure can be more successful at hooking and sustaining Hook Factor than a Composition that does not employ those elements.
The 'Enough' concept is the Songwriter's judgment call. How much Introductory Movement is 'Enough'? How much is 'Too Much'? How much is 'Not Enough'? With experience a Songwriter should begin to sense the timing of components, sensing when Enough Intro has been given and it is 'time' to begin the main body of the work. You should sense when Enough Verse has been exposed to set the scene and it is 'time' for the Chorus. Try your Songs with the Chorus to start and see if that works, in your judgment call. Examine the Verse Lyric and see if there's 'Enough' exposition, or 'Too Much', or 'Not Enough'. Remember, "Don't bore us! Get to the Chorus!"
How much Repetition is Enough? How much Change is Enough? Too much? Not Enough? A Song that is continually Changing lacks Structure. A Song that is continually Repeating gets monotonous; mono-tone-ous. Either can allow the listener to become 'unhooked'. They drift off to other thoughts, stop paying attention, and only realize it when the Song ends and they realize they haven't been listening, and may not be interested in listening again.
Terrestrial radio play used to earn Royalties, making money for the Songwriter(s) and Publisher. The reason they played music was not to promote the art; it was to keep listeners 'hooked' until they could play the commercial ads that pay the bills. Short Songs left more time for commercial ads. Songs used to be two minutes long. It is surprising to listen to those Songs now and realize they do not seem 'short'. They get done everything a Song needs to get done, in their two minutes and a few seconds. They are 'Enough'.
Who is 'buying' music now, and how? How are 'consumers' 'consuming'? Where is the market for your product? If you can supply their desires and demands you can sell in that marketplace.

There will always be another song to be written. Someone will write it. Why not you? www.garyeandrews.com