Everything takes longer at the beginning, so the first four ebooks you write will take you the longest, because you’re learning as you go. Once you build your writing muscles, and know what you’re writing, you’ll write more, and more quickly.
Please be prepared: know that you’ll be SLOW when you get started. I assure my writing students that it doesn’t matter how long your first ebook takes you. You’re learning when you’re writing your first books. No one is born knowing how to write: you need to train your brain, and practice.