I'm an iOS developer for a number of enterprise & public facing Apps on the App Store, so I can give you a pretty good indication of the process needed to update an App for iPhone 5 support...
1) Graphical Work...
This is the first step, and to begin with means creating new splash screen images at the very least. Without these the App will run in letterbox mode regardless. From there the amount of graphical work really depends on the App. Any full screen graphics will need to be re-done and devs may choose to break these graphics into several pieces and make it stretchable (Especially considering all the iPad mini rumours) to allow support for more screen resolutions at some point down the line. Games etc will will obviously require much more graphical work to support a 4" screen.
2) Dev work...
I haven't been any more specific about this, because it's nearly impossible to cover all scenarios. Each app will require a slightly different amount of work to support a 4" screen. I will tell you, the easiest apps to update are the ones comprising of a top section (navigation bar), a bottom section (tab bar) and a stretchable middle section (a table view or map view for example). If you take a look, with a few exceptions, most of the Apps that are iPhone 5 compatible (like Tweetbot, etc) have this stretchable middle section, it doesn't really require any graphical work to make the change just a few layout / constraint tweaks to make it work.
Other apps will require SIGNIFICANTLY more work, especially if the developer is working to provide a unique experience for the iPhone 5 (something Apple wholeheartedly recommended in the keynote) so I think the point to note here is that while these Apps might take a bit more time to update, they will be much better to use on a 4" screen when they are released.
3) Testing....
Any developer worth their weight won't release an App until they have tested it thoroughly and completely. In some cases this means writing code to test their new code (known as unit tests.) Generally these would focus less on the graphical aspects of the App and more on the functional logic behind the App. But if a dev is doing specific things for an iPhone 5 you can bet they'll want to write some unit tests to make sure the code is functioning as it should. If these tests fail, you go back to step 2 and make it work before restarting this step.
4 ) On device regression testing...
This is generally the final test before submitting to Apple, and again any Dev worth their weight will want a full range of devices to do this test on. So if a dev is having trouble getting their hands on an iPhone 5 (not all devs can line up on day 1 to buy the device!), this will delay the process. Sure we've got a simulator, but Apple highly recommends testing on a device before submitting an update and I know i wouldn't even consider an update if it wasn't tested on a device beforehand. Like with point 3, if any bugs are found in this stage, you go back to step 2 and make it work. Then you go through step 3 and step 4 again.
5) Submitting to Apple...
So once we finally have a build we've tested and are happy with, we can submit it to Apple for Approval. This process can take anywhere up to 2 weeks generally for an update (sometimes longer if it's a new App) and if Apple finds anything not to their liking, it gets rejected and back to step 2 you go... then step 3, then step 4... etc etc (are you getting it yet
It's not necessarily as easy as Apple suggested for us to update all our Apps to support the 4" screen!)
So that's a general overview of the steps it takes to update an App to support the new screen size. I'm not in any way saying it is the same for all dev's and all Apps. For example, games etc will require significantly more graphical and dev work to support the new screen size.
Happy to answer any more questions you may have!
