Recently, we got some new photos of J-15S coming out, which is always exciting. J-15S is the twin-seated version of China's naval flanker J-15. Its prototype flirst flew from SAC airfield on November 2012. All of the J-15S photos we've seen so far have shown Taihang engines powering the aircraft. That would suggest J-15S will be powered by Taihang right from the time it enters service and that the single seated J-15 could be powered by Taihang after the first batch.
We know that J-11BS entered service relatively soon after J-11B entered service after a relatively short flight testing program of around 2 years. It probably could've been even shorter if not for the problems with Taihang engine at the time. We saw very few photos of J-11BS in flight testing. In comparison, it seems to me that J-15S will be having a longer flight testing program, although not as long as would be expected out of a new variant like J-11B or J-16.
Going forward, I think it is likely that J-15S would be used in more roles than just as twin-seated trainer. First of all, J-15S could be developed in navy's version of J-16. The J-15 airframe should already be strengthened to handle the punishment of taking off and landing on a carrier, so J-15S may not need too much additional work to handle the additional payload expected out of a strike fighter. J-15S would not have the range or payload of J-16 due to restrictions of taking off from a STOBAR carrier, but it could be installed with similar avionics and combat system for strike missions as J-16. It will be able to carry different types of anti-ship missiles, anti-radiation missile and ground attack munitions/missiles that can be launched by the pilot in the WSO seat. Having the second pilot should make J-15S a more effective strike aircraft than J-15. Aside from just anti-shipping missions and ground attack missions, it could also be fitted with the subsystems for SEAD missions and EW warfare. When Su-30MK2 came out, there was a lot of talk about its usage as a "mini-AWACS". While that is a little overblown and CV-16 will already have several Ka-31 and Z-18 AEW helicopters, J-15S can certainly be equipped with the necessary avionics to process data from a group of J-15s and direct their actions.
In summary, J-15S will be a very useful naval aircraft for China's burgeoning naval air arm. It will likely have many use beyond that of a trainer.