Back in the 60's and probably even earlier, the rules were the song could not exceed 2:30 in length or it risked not being played on radio.

Some of the bands made 2 versions...album cuts and radio cuts to get around the rules. The 45 was the radio cut.

Now, the rules are a bit different. Songs can be longer but the SHALL NOT EXCEED length has moved out to 4 minutes. Most still fall in the 2:30 to 3:00 length.

The new rules include: (or so "they" say)
Intro not longer than 13 seconds. Shorter is better.
Be into the chorus before 60 seconds
Keep it under 4 minutes max...less is better.
Keep it uptempo, (despite the fact that the majority of hits are ballads)
It must sound "radio ready" which means professionally done. (this one is true) The people listening and making the decisions have no imagination and no idea of what a song could be,,,,so the song should have the parts in it...intro, guitar, piano breaks worked out and polished, singers that sound like the stars, and the entire song should sound like it is ready to go on the radio "as is".

I don't know how many times I have heard these things repeated ad-nausium at songwriter workshops and seminars. Some are true, some are good advice and some not.

Of course, the rules are there but established artists can set or break the rules as they see fit depending on their clout.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.