This has not been covered by the series so yeah...
How long will this 52 Floored Tower, nicknamed the Prudential Center, last after humans?
5 Years: The Prudential Tower's windows either fall or break due to a small (but gusty) wind storm. But the winds won't destroy the rusting Prudential Tower.
Around 6 - 8 Years Steel beams holding the observation deck are heavily rusted and break. The observation deck collapses onto the rest of the building below, destroying the Prudential Tower in a cascading fall.