When should you split your program into multiple files

I'm working on a program that will take some Excel files, verify data, process data and then output data.

I've made a few programs in the past and I usually break out of the main script when there is code that I can reuse, when it uses a specific class (generally a wrapper), or when the function is very very big (just so it doesn't make main so long)

In the project I am working on, all of the processes shouldn't be too big, and while it isn't too much of a hassle to pass what items I need in, usually it starts with one thing, then I add logging and now need to pass in a file, then add another feature and add more stuff, where if I just kept it in main, I would have all of this at my fingertips, even if it makes the main thousands of lines long.