He’s a field reporter. He goes out into the world to write about stuff. If he’s gone for three weeks working on a story, that’s what it takes.
He’s also super smart and super fast, so he can write the story in a small fraction of the time as most reporters.

So, as Superman, he flies to where the problem is, solves it, then writes about it as Clark Kent.

Same thing Spider-Man does, except a big part of it is also getting pictures of himself as Spider-Man doing the problem solving.