McCartney has been writing and recording from 1958 to present, and like Prince, he plays multiple instruments, although Prince is the better musician. But he writes songs all the time. Not necc. records them, but has them written down and prob. does some shorthand on melodies and chords. The guy has natural music talent, without a doubt. It might not be all great, but it comes to him naturally.

Elton John is another case where music comes quick and easy to him, even though he doesn't do lyrics. He's slowed down on recording.

In Prince's case, when he gets ideas, he has a studio at his disposal 24/7. In the case of everyone else, they might get their ideas down at different times, and bring that all with them, and work it out in the studio within a timeframe.

Bob Dylan spends more time on the road than in a studio.

But Stevie Wonder, I have no idea how much material he has, I'm assuming there's a ton of demos, outtakes, finished but unreleased recordings..