I Decide How Much Time it Takes

I don’t understand when we say in scholars that “we decide how much time something takes.”

If I have an outcome that I’ve decided on, sometimes I don’t know how long it will take. For example, I have an evergreen funnel that I’m trying to launch, and there’s a whole bunch of back and tech stuff that needs to be connected. I legitimately don’t know how to do that. And I have no idea how long it will take to get it to work.

Can you explain how the idea that we decide how long a task will take applies in this circumstance?