How long it takes you to complete a project naturally depends upon many factors. What you are writing, its expected length, the amount of planning and research you put into the project before writing, plus other factors. But the short answer is, it shouldn’t be quick; it should be longer than you think it should be. But there is no specific answer, as it will vary.
New writers often imagine a story will come to them, that it will flow from brain to fingers, and in no time, the story is done. It is true that it sometimes happens. A songwriter will wake up in the middle of the night with an idea, and in an hour, have come up with the lyrics and melody. A screenwriter writes a hugely successful script in two or three days. A writer proudly announces they wrote a novel in three weeks, and it ends up selling well. A student writes a paper the night before its due date, with little research, and they get an A. So, sure, there are exceptions. But even putting aside the question of the real quality of these writings, this kind of quick, “it all just came to me” projects are very much in the minority. Don’t depend upon it happening to you, and you should probably not trust the feeling even if you get it. When possible (and that should cover most writing projects) one should take their time.
For the majority of writings, you should carefully think of what you are doing.
For academic papers, for example, what is your paper topic? What sources (and how many) are you using? Is it supposed to be a one-page essay, or is this a 100+ page dissertation? Have you written an outline yet? [See the blog post on Supplemental Tools.] All of these things, plus most of the research, should be decided and laid out before writing even begins. And then, as you write, remember it should be a first draft only. [A similar approach would be used for non-fiction books. You should know what you are writing, the layout, the length. Only after collecting that information, and doing and collating your research, should you be writing.] And while procrastinating is every student’s burden, the more time you give yourself, the less you need spend in any given stretch, and the more time you will have to hone and proof across multiple drafts before submission.
For a story, your approach will be slightly different. The research will be more about world-building, and about the technical details of the plot that you may not already know. But notice research still needs to be done. The layout is about keeping character and location details clear, as well as plot details, or what comes before what. Again, notice it still needs to be done.
A good time rule is to aim for five to ten pages a day (a page equals about 250 words). Some even advocate a single page a day. How much depends upon whether a given set of paragraphs are dialogue, exposition, or the long descriptions that are part of world-building. The first two typically go faster if you know your plot. The world-building can be detailed enough that you might indeed only get a page, maybe two, done in a day. Don’t worry, this is perfectly normal. Also, while you should not expect an entire project to just flow, there will be days you are just “on” and you might as well take advantage of them. Because there will also be days when you are most decidedly “off” and there isn’t much you can do about that except try to push through. NOTE: Just remember that like with academic papers, your initial story is simply (or should simply) be a first draft. Don’t get hung up on perfection the first time ’round.
Don’t worry about the average time it takes writers to complete, just concentrate on you. That said, most best-selling writers churn out a book a year, often with the help of co-writers to make that turn-out possible. I believe this is too fast, as that includes initial ideas, research, writing, editing, proofreading, all of it. But I also understand we live in a world where most readers forget about someone if they don’t publish often. Some of my clients even publish twice a year, but these are self-published, a world even more demanding in the public arena than publishing houses. Just remember that the more you write, the more watered down its quality may well be.
NOTE: The above advice on fiction is for full-length books. For novellas, short stories, or flash fiction, obviously more can be written in a year’s time. The same is true of school papers. The shorter the project, the quicker it will be done. Perhaps obvious advice, but worth pointing out, anyway. Good luck on your project.