I've found that a good rule of thumb is the "3 rule". When you start using the same code in more than 3 places, put it in a shared lib type of file / folder.

I've found that a good rule of thumb is the "3 rule". When you start using the same code in more than 3 places, put it in a shared lib type of file / folder. When you start using the same code across 3 projects, put that into a "module" and manage independently in version control. Depends on the language / framework you're using, but I've found that general rule pretty helpful in my experience.