*ahem*
In recent times for NRS there has been 2-3 years between games, if we were to accept the following:
- That they are currently working on MKX/10 (we know they are working on something)
- That they aim for a 2015 release to coincide with the next Mortal Kombat film (source required)
Then we can theorize that 2-3 years after that we could maybe see the release of Injustice 2, so 2017 or 2018.
However, it should also be noted that developing for a new generation of consoles could take longer than previous games, that the tie-in to the movie could cause release date issues, offices can get destroyed, projects de-railed, all sorts of weird shit.
Could be longer, could be less.
EDIT: Forgot to mention, there is no guarantee after MKX, they might do another MK, or MKSM2, or the other MKSM2, or something completely different... like Harry Potter vs. Pacific Rim... not straight to the next Injustice.