The setting is some indeterminate amount of time in the future. Humanity has had a rough recent history being rocked by wars, disease, and natural disasters and the current iteration of civilization is a shadow of what we know today. Most history has been lost and the knowledge of technology and science has been fractured and split. Heavy caliber machine guns are mounted on oxen drawn wooden carts. Coal fired mechanical forges create ceramic containers that people use to store the game they hunted for the winter. A fairly typical post apocalyptic environment.
One piece of the Old World remains however: stunningly enormous collections of spires that seem to pierce the sky. Clusters of them stand like giant artificial forests in what clearly used to be inhabited lands, but where no one has lived in existing memory. Some of these towering cities are by the coast, now standing in an ocean bay that rose up with the sea to consume them, some are inland at the bottoms of natural basins now overgrown with unending forests. The people of this new world almost worship the amazing beings that must have created these enormous structures and mourn that this era of their history is long since past.
Question: How many years from today could this realistically be while keeping the cityscapes recognizable for what they were?
Assuming contemporary cities built using modern construction techniques and materials, how long could a place like Hong Kong, London, New York, Atlanta, or Tokyo be expected to be recognizable as cities? I'm specifically interested in how long the taller buildings might last, structures that would create a "ruined skyline" effect.
They don't need to be livable by any means, nor do the buildings need to be structurally sound (in fact collapses could be a good thing, plot wise). Think how today we look at the ruins of ancient Rome and the Parthenon in Athens: definitely ruins, but clearly recognizable for what they were and testaments to the skill of their builders.
I'm not looking for an exact number of course, but something at least in the order of magnitude. Could I say this is five hundred years in the future? A thousand? Or is only a single hundred years more realistic?