If they are properly maintained, structural engineers believe skyscrapers will last forever.
After proper design, the key is the integrity of the materials.
Steel or concrete can be damaged by things like acid rain, but if damaged material is replaced, the building will survive.
If a structure like the World Trade Center were abandoned and not maintained at all, the first thing to go would probably be the curtain wall, the skin of the building.
As it was weathered by rain, pieces would probably start to fall off.
However, it would probably take twenty-five to fifty years for the core to be affected.
Massive buildings of masonry may have a longer lease on life than those with a glass skin, but advances in sealants and other materials and techniques mean modern buildings may have more staying power.
Except for disaster damage from things like earthquakes, bombs and fires, the reason buildings come down is often economic, because there is no money for maintenance or because the real estate is too valuable for the structure that is on it.
In a throw-away economy, we throw away buildings, too.