Not as long as you would think. Non-reinforced concrete only lasts a few generations, whereas there are Roman concrete roads still in (mild, rural) use today.
We actually know why too. Recent science shows that Roman concrete used a slightly different formula that took much longer to set -- weeks or maybe months rather than days. They didn't know how to make fast-setting concrete like we have today. But the length of time also made the structure stronger. We could build that way, if we wanted. But we don't.
[1]http://www.sciencemag.org/news/2017/07/why-modern-mortar-cru...
These days, we're so crammed in where ever there are usable resources, that the moment we die, or move on, someone claims the space and adapts it to their needs. Space is at such a premium that it's illogical to lock it out as one particular usage for centuries.
Often times buildings are disappointingly short lived, but I think sometimes people lament it a little too much. If we need a different building there 50 years later, and we built it to last 200 years, then a lot of material and effort was wasted. There's often a lot of good that comes from knocking a building down and using the limited space for something more useful.