Short answer : anywhere between 35 to 75 years but on an average – 60 years. The property still requires relevant and regular maintenance activities that add up to expenses. Moreover, after 90 years or 99 years, the flat would have to be vacated and disowned or the lease will have to be renewed according to the new rates.
Long answer: All large buildings are now designed using the probabilistic approach and with the help of a lot more quantized knowledge and stats, it’s tough to put a number on the design period. Moreover, the life of any building for that matter also depends on the kind of management done with it including inspections and maintenances. Something made for 70 years can fall in 15 years is proper management is not done. Also, if we fish to put in more money than planned we can actually carry the building forward to stand more than 70 years by repair, reconstruction, and retrofitting. But, coming back to our question, the best estimate would be 60 years and not more. Further, a lot also depends on natural events. Let alone regular monsoons, this year we’re having floods like those in 2013, so a lot of damage is expected in the real estate industry.