Jack London, John Barleycorn
Most other writers probably fall somewhere between these two extremes, but I suspect the majority have more in common with Hemingway than London. Writers I read about or hear speaking on the subject at book festivals or writers' conferences usually set aside a few hours each day, most often in the morning, to do their writing. Writers with full-time jobs may get up early to write a couple hours before going to work. Women with families may write before their kids get up in the morning or after they go to bed at night. Some writers try to get away to Starbucks or a public library to write for a couple hours.
Few writers seem to spend all day writing, although I imagine some do. I don't know how prolific authors like Joyce Carol Oates or Isaac Asimov can churn out so many books in one lifetime without spending many hours each day with a keyboard in front of them.
However long a writer spends writing each day, two bits of Hemingway's advice strike me as particularly sound. First, stop at a place where it will be easy to get started again, rather than stopping when you're stuck. You don't want to take a chance on letting a good idea get away from you while you're taking a break, but halting in the middle of a paragraph or even in the middle of a sentence, after you have already gotten something of the idea down in writing, seems like a good way to get a quick start the next day.
Second, I like Hemingway's suggestion of letting one's subconscious mind do the work. Sometimes our best ideas or the best solutions to our problems, whether we are writers or not, come when we are not even thinking about them. I recall Francine Prose calling computer solitaire "the dirty little secret of the literary world." She, as well as many other writers she has talked with about it, spend a lot of time playing computer solitaire when they are supposed to be writing, she says. Perhaps this is a serious distraction for writers, yet I haven't noticed any slacking off in the writing and publication of books. Writers seem to be as productive as ever. Is it possible that computer solitaire may, in fact, give writers a chance to let their subconscious minds do the work while they have a little fun?