How long will this project take? That depends on how you count.
Let’s talk about the two parts of creation.
There’s active time – how long it takes to write the notes down, to actually craft the final product. That might take a rather small amount of time. I can write a choral arrangement in 3-4 hours; I can write a daily post in 20 minutes or less.
But there’s passive time, too. Time when you’re creating but nothing is changing. I need passive time throughout the day to determine what I’m going to write. I need hours of passive time listening, absorbing, thinking, singing in my head and around the house before I am ready to write the arrangement down. I needed years of passive time learning my craft, and the next project might need significant passive time learning a new skill or tool.
All that passive time counts, and it should be part of your equation for how long you need to complete a project, for how much you charge to complete a project, for how much you value your own creations.