I can give you a parallel from when I was IT Manager at BMW. The labour is charged in FRU (Flat Rate Units) this is a unit that is derived as an average and a dollar value is put on this. BMW states (for eg.) that to remove the engine from the car is 5 FRU's worth, it does not mean it is 5 hours, but depending on the FRU time and Dollar value of that dealership, this will translate into the corresponding time and dollars. This is determined by the overheads and the skills of the technicians at the dealership.
Similarly, you can have an app made in say 3 units, but these units will be different for every developer or organisation. The old methods using Lines of Code etc are not very relevant anymore. I made some of my initial apps including art in Objective-C in less than a week (while working full time), they are still solid but would I want to do them again differently, maybe yes, Will it take the same time if I used a lua based framework, maybe not or maybe yes, if I do not find proper documentation on a particular API that I might want to use.
Lastly, if someone is full time into app development, then sometimes it takes longer as they have the luxury to have it modular and technically better than if you wanted to get an app out in the limited time that you can invest into it, output takes priority.
You might have seen many games being made in 48 hours (Ludlum Dare) so everything is possible... but if you were making an app like Angry Birds, you would work on the Graphics as a project (they would keep evolving over time, before you feel just one more iteration and they will be perfect) while the developers will work on the code and make that slingshot go as realistically as possible and so on. The developers would talk about creating an engine, a 2D/3D rendering pipeline and that is where the bulk of the time for large studios is taken up. In some rather rare cases they end up reinventing the wheel.
From my exp. as a FT mobile development studio, there is no such situation of everything in palce. The moment you start to get it all together, the client will request changes to the graphics, gameplay, etc.
Would like to hear others point of view on this.