Let's assume that the choice was between putting the FFXIII team on this or on another 2 year console project after Lightning Returns shipped. And let's assume that the console project performs about as well as LR did in Japan - which puts it at about 500k units sold, and S-E making maybe 50 bucks per copy. That would be 25 million dollars of revenue. How long would it take Mobius to surpass that? A month?
Hard to say.
First of all, we do not know how much the app is grossing; the ranking is not enough to say something meaningul, in my opinion; taking apps monthly revenues from financial reports as a reference is a bit reacing, in that there might be huge seasonal effects and daily distortions due to inherent factors (I suspect during weekends people spend more) and endogenous ones (events, promotions). Secondly, we have to remember that Apple is taking a 30% cut on all sales generated through the App Store and the game itself (so million a day would translate into 700.000) + featured advertising (which might involve costs or not).
As for a physical release, there are other factors to account for. For example, DLC; we know that paid downloadable contents are popular among gamers, and they might be a good stream of revenues on top of the entry fee (I suspect Theatrhythm sequels were greenlited thanks to the success of DLC, because sales were not that high overall). Also, worldwide release; we have seen how many Japanese games have struggled overseas, while doing spectacularly well in Japan; therefore, a FF game by the same team might have been developed with Western markets in mind (LR bombed in the West as well, but at least XIII-2 sold good enough to grant another entry).
Therefore, it is quite difficult to answer your question, and maybe also pointless. After all, whether a success or not, SQEX will keep developing on mobile, otherwise we would have not gotten Mobius and Record Keeper to begin with.