It really depends on the app's core design. A "well planned for the future" app, it should be relatively simple. A "hard coded for two fixed resolutions" app, it will take a lot more work to add the new layouts; especially if they go to add new iPhone 6 Plus landscape mode functionality, like lots of first party apps now do.
My company (among other products) develops mobile apps for our clients that get rebranded for each client. As soon as the phones were announced, we immediately insisted that all of our clients send us updated splash screen graphics at the two new resolutions, along with "3x" icon graphics for the 6 Plus. But some of our clients take a while to get that to us.
So even if we the developer were 100% prepared, there is the delay in how long it takes our clients to get us the updated graphics. Some clients take a *LONG* time.
Then there is the actual app-side coding. We did try the simple "change the development target" for iPhone 6 and 6 Plus, and did find quite a few bugs. Parts of the screen that should auto layout don't properly. Buttons that should be in line with each other aren't. So the developers are going through with a fine-toothed comb to find any other bugs. It takes a while. We had "fully iOS 8 optimized" apps on iOS 8 launch day for our clients that had given us all the pieces we needed. But iPhone 6 and 6 Plus apps are taking a while longer.
Our iOS development team is in New Zealand, and even though we had multiple employees wait in line on 6 Launch Day, they were only able to get iPhone 6s, not 6 Pluses. So we had to buy 6 Pluses in the US and ship them over to NZ. So that added a few days that they couldn't fully test, either.
Apple's backlog is pretty bad right now. For instance, my favorite RSS reader is Newsify, and the developer says he submitted his iPhone 6/6+ update on September 29th, and it is still waiting for review.