If it’s poetry you’re writing, your target is probably 50 to 70 pages.
Estimates vary about how long a novel should be. According to Deborah Ritchken, an agent at the Marsal Lyon Literary Agency, “There really is no set rule but the average novel runs about 110,000 words. Obviously, there are novels where the word count runs much higher!”
If you’re J.K. Rowling, then the sky’s the limit. (Some might argue that the later Harry Potter books needed a good editor.) A book of nonfiction should also clock in around 100,000 words. I would think a collection of short stories should be closer to 60,000 words.
Whatever the genre, you should write much more material than you actually need. You want to have the luxury of cutting work that isn’t up to the quality of the rest of the book. For my collection of poetry My Mother and the Ceiling Dancers, to arrive at 50 pages of published poetry, I wrote about 135 pages. I cut two thirds of it. That wasn’t easy for me. There are still poems in that 85 pages of crossed-off material that I get a twinge of regret when I read and remember that I couldn’t include them in the book. But I know the book is better off without those deleted pages.
The trick is to write more than you need but not to get so attached to what you write that you refuse to part with the sections that are weaker, or extraneous to the direction of the book as a whole. Ultimately, it’s the reader’s experience you have to prioritize, not your affection for your own words.
All writers create bad drafts, and/or writing that is not up to their best work, or doesn't mesh with a current project. The challenge is to keep that work somewhere private, rather than try to publish it.
I have a file I call “Uncollected Poems,” which is my euphemism for poems that never made it into any of my books. Ultimately they didn't make the grade or fit in the collection I was working on at the time. I’m not throwing those poems out. Some of them I continue to polish. I enjoy revisiting many of them. I occasionally will send one or two out to magazines or anthologies. But I realize they don’t belong in any book I’ve written to date.
In the literary world, the quality of your work is judged not only by what you publish, but by what you don’t publish. If you establish a consistent standard for your writing and keep to that, readers will see that sheen in your work, and be drawn to it. If you don’t keep to that standard, you risk being judged by the worst of your creations.
Other recent posts about writing topics:
Putting Together a Book Manuscript, Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8
How to Get Published
Getting the Most from Your Writing Workshop
How Not to Become a Literary Dropout
Working with a Writing Mentor
How to Deliver Your Message
Does the Muse Have a Cell Phone?
Why Write Poetry?
Poetic Forms: Introduction; The Sonnet, The Sestina, The Ghazal, The Tanka
Praise and Lament
How to Be an American Writer