If you're doing iOS development and have previously only done non-business apps, competency could be demonstrated by being able to architect a non-trivial app. The lines of code don't matter as much as how you manage the code (low coupling, strong cohesion).
Unfortunately, the non-business apps may hold you back from a portfolio standpoint, especially since you said there was nothing complicated in their inner workings.
It might be worth your while to work on a more serious/business-oriented app (that scratches an itch of yours - not something that you have no interest in). Something along the lines of something that reads from an API and displays it/allows the user to do something with the data. Throwing in some nice visual eye candy with UIDynamics or the older CoreAnimation is a nice touch, as well.
In general, during the course of the interview, since you have a non-traditional background, you want to demonstrate that you took on technical challenge(s) and solved them. Since your apps to date appear to be not too complicated, please consider building something complicated in the month you have allotted yourself - just for the experience of it. The bar keeps getting higher with each successive iOS SDK release - at this point, you should be comfortable with storyboards, blocks, not blocking the main thread, handling async stuff, networking, and the many open-source libraries out there that make things better for iOS developers.
For bonus points, build the app all in Swift (and then you can tell the interviewers about it - most developers have not seriously delved into Swift for lack of time/bandwidth).
Also, start reading the articles curated by http://iosdevweekly.com if you have not already
Good luck!