Personally, even for a prototype, I'd be using functions immediately as soon as I saw (or anticipated) I needed to do same thing twice - mainly so that if I want to change it later there is one place to change, not many. It's the same for production code of course, but when prototyping the code structure may be quite fluid and you want to keep making changes easy, not have to remember to update multiple copies of the same code.

I'm really talking about manually writing code, but the same would apply for AI written code. Having a single place to update when something needs changing is always going to be less error prone.

The major concession I make to modularity when developing a prototype is often to put everything into a single source file to make it fast to iteratively refactor etc rather than split it up into modules.