Here's what Apple has to say about it in part:
On Mac computers that play a startup sound, you can release the keys after the second startup sound.
On Mac computers that have the Apple T2 Security Chip, you can release the keys after the Apple logo appears and disappears for the second time
Unlike what I was used to seeing on my Intel Mac mini, on my M1 Mac mini there is only one startup chime and the Apple logo appears only once, but holding until the progress bar appears works on both Intel and Apple SoC.
