What is the golden rule for when to split code up into functions?

It really does depend on the size and scope of your project.

I tend to split things into functions every time there is something repeated, and that repetition generalized/abstracted, following the golden rule of DRY (Do not repeat yourself).

As far as splitting up classes, I tend to follow the Object Oriented Programming mantra of isolating what is the same from what is different, and split up classes if one class is implementing more an one large theoretical "idea" or entity.

But honestly if your code is too fragmented and opaque, you should try considering refactoring into a different approach/paradigm.

If I can give it a good name (better than the code it replaces), it becomes a function