I would say reading and writing are the “greatest” parts of a writer’s time but I get what Samuel Johnson is trying to say. A writer must read and read extensively in order to learn how to write and how to tell a story (or do whatever it is they’re writing). One book just won’t be enough to learn how to tell a really good story. It takes time and lots of research. I think I read many many books for two years before I wrote my first novel. And even though I’m in the process of writing another novel I still feel the need to keep reading, especially in its specific genre. Of course, I’ll never stop reading because I want to keep learning how to write the most effective and mesmerizing book I can. And all this reading is time well spent.