I think they bought a significant quantity of Russian AG munitions over the many Su30 purchases, since at the time China was buying with war in mind given the Taiwan crisis that propelled PLA modernisation into high gear. Who knows how much is left?China doesn’t even have that many Russian AA munitions left. Only legacy flankers still use them.
Something else to consider is that the fact that those munitions have finite shelf lives that they are fast approaching may be another reason why the PLAAF is in no hurry to MLU their Su30s - they may well have decided to get as many years use out of the Su30s as possible since they can be used to give pilots invaluable experience with using live munitions without running down inventories of newer and better Chinese made smart munitions.
Remember that training is a perishable skill, so you don’t want to use your stockpiles wisely to give as many pilots experience as frequently as possible. As such I imagine the PLAAF would have drawn up a detailed multi-year schedule of how much munitions to use each year to maximise their training value over as many years as possible.
Once those munitions stocks start to run low is when I would expect to see the PLA start to upgrade their Su30s with modern Chinese radars and avionics (so no more support of imported Russian stuff). There will still likely to be a significant proportion of munitions left when this starts, as the upgrade of the whole fleet may take years and the PLAAF would not want to have such an important part of their fleet essentially toothless for so long.
It would not surprise me if a decide part of the fleet is never upgraded and just kept around in this training role and eventually retired once all the old munitions have been used up.