The design life of an RCC structure is generally taken as 75 years.
But that may increase or decrease based on the following factors. It is dependent on the quality of construction, quality of materials used, weather conditions, seismic conditions, maintenance of the building, etc.
From the question, when buying apartments, the builders will not disclose the genuine construction practice that was followed. So in order to be sure, it is best to test the structure using Nondestructive tests.
To know more about the nondestructive tests used to find the strength of the structure,
https://theconstructor.org/structural-engg/estimating-life-of-a-residential-building/369/18