The chorus of a song is a big deal. It'll make or break the greatness of the whole thing. The chorus likely the most memorable part of your song and you need the length to be perfect. Not too short, not too long. If your chorus is too short, no one's really going to remember it, and if it's too long, it won't be catchy enough.
In theory, your chorus can be as long as you want it to since it's your song. But if you're trying to get the approval of fans and the music industry, you will probably want to follow the industry standards when it comes to toy your songwriting in order to get the best results and the most pleasant response from your listeners.
To understand chorus length, you will first need to make sure you understand a bar or a measure of a song, and a song's BPM.
A bar, also known as a measure in a song is a small section of a song that usually accounts for 3 or 4 beats each. All songs are made up of dozens of measures, also called bars since they are literally separated by bars in sheet music. The rhythmic feel of a song bar makes it feel like they should naturally be paired in 4s, i.e a chorus that is 4 bars, 8 bars, 16 bars, etc.
A song chorus usually lasts 8 or 16 bars in a song. Your choice to make the bar 8 or 16 depends largely on the chord structure of the song, the lyrical structure of your song, and most frequently, the BPM of your song.
The BPM or beats per measure is a way to measure the tempo of your song. Songs that are slow are usually less than 100 BPM, while fast songs are usually over 120 BPM. A fast song is way more likely to get away with a 16 bar chorus in comparison to a slow song, in which the chorus may seem to run too long.
If you have a pop song that has a very simple song structure, you will likely want to have a shorter chorus so that it doesn't get way too repetitive. If you have a long chorus with the same chords and the same melody happening over and over, listeners might get annoyed.
Similarly, if you have a pop song that has very simplistic lyrics for the hook, with one phrase or less, you will also likely want to make your chorus 8 bars instead of longer. This is not to be confused with a chorus that repeats towards the end of a song, that's totally okay!
If your lyrics are a bit complex and you want to focus on a few phrases for your chorus, you should go for a 16 bar track.
Verses are usually around 16 bars of a song. A lot of people like to make their choruses and verses the same length to keep it simple, but it still works really well to have a verse that is 16 bars and a hook that is 8, especially if you have simple lyrics in the hook.
A song Bridge is often just 8 bars. The bridge can stand alone or be followed by a breakdown, but the bridge usually brings new melodic ideas to a song that would change the song up a bit too much if it were too long.
The Intro and Outro in songs are also usually 8 bars. There's nothing wrong with having an intro that's half the amount of an outro, or no intro or outro at all, but most songs have at least an intro in order to slowly reel you into the song and not just plop into the verse or chorus out of nowhere.