I suppose it depends what you want the app to do and what you’re using to build it. The simplest app you can possibly make using something like Ruby could probably be made in about 15 minutes, but the apps you use every day on your smart phone probably take weeks or months to build and get everything working correctly.
I’m guessing you are talking about mobile apps. Web apps are easy to create using some tools. You can learn how to do a simple voting app on this website: https://docs.djangoproject.com/en/dev/intro/tutorial01/. However, if you want to do mobile apps, you will need to learn some programming languages like Java or Objectiv C (iPhone). To make simple apps, it can take a few days. More complex apps that require fancy graphics, server-side communications, or complex logic will take a few months at least. Most good apps are developed by a team… but read this guy’s story… quite inspiring! http://en.wikipedia.org/wiki/Nick_D%27Aloisio