A novella is between 15-20k words. I recently spoke to a writer who said she used to write 20k words a day! her family had to have an intervention because she was not sleeping 😛 But can you imagine, you would have a book done in a single day.
For most writers between 2-5k a day is usually a good goal. During slow periods it might be 500words a day just to keep things going. But what you have to remember is that this is just the writing part. The whole process of revisions and edits can take months after that. Even the most seasoned writers will need at least 2-3 revisions and re reads before they can even think of submitting it. Why? When you write a 50k book for example, there are a lot of chances for your continuity to get thrown off. You might start with a brown haired heroine then accidentally change it to blonde. Or you might mention your heroine has no parents then accidentally mention a mother in America later on. There are 50,000 words to check and make sure each and every sentence makes sense. This takes a lot of effort. Trust me.
In fact writing a book in a day is possibly a really good way to end out editing for the next two months. The slower I write the fewer errors i seem to come up with. You have time to think it over, come up with new twists and turns and make it more in depth. Sure its fantastic to get to the end of a good session and realise that you have just plowed out 10k, because you are that much closer to the end of the book and there is nothing quite to satisfying as finishing writing a book. Even finishing edits for me isn’t as satisfying because i always know i could read it again and edit some more but there comes a point where you just have to let it go.