I had a glance at the app store description for Prayer Wall. It seems it lets you submit prayers, browse prayers others have submitted, comment and flag prayers.
How complex it is depends on how much programming you've done before. If you haven't done any programming before, add in another six months in order to get up to speed.
First you need a backend server to handle all this. It could be written in Django (a Python framework), for example. You'd need to handle registering different users of the app (based on their device ID), a secure connection to request data and to send it back. You'd need a database running to hold everything, and an API to allow the device to send the requests it needs. For an experienced web developer, that's going to take a few weeks.
To do the iPhone app itself, you'll need to use something like Core Data to keep hold of the prayers and comments that are currently on your device. Then you'll need some way of querying the server to see what's changed recently, using the API you wrote. You'll need to download the new prayers and comments in a background thread, so as not to make the user wait for ages every launch. But because Core Data is a bit weird about multithreading you'll have to set up notifications to your main thread to get your UI to redraw itself when the data has been downloaded. You'll have to think about what to do if some data the user is currently looking at is amended. Then there's writing the UI itself. A lot can be done with the standard navigation and table view support, but you'll need to subclass the table cells to get them to display in your own colour scheme. The wall itself is probably an entirely custom view.
I'm not very good at budgeting my time, but this seems like a reasonably advanced app to attempt if you're a beginner. As an experienced dev, I'd want to commit a few months to it, especially if I was also writing the web backend. If you already know some programming, learning the Cocoa APIs while you write your first app is hard but doable. If you don't, I'd recommend learning Objective-C and Cocoa first (via a course or a couple of books) before launching into a project like this.
Amorya
