My understanding is that modern concrete would last as long as Roman concrete would, if we built the same types of designs that Romans used concrete for. The big difference is that we want to span gaps without using large unwieldy arches, so we need tensile strength, so we need to use steel reinforcing bar in our concrete, which is the eventual pathway to failure. (Well, that and heavy machinery like semi-tractor trailers, which the Romans also didn't have to design around)

The Romans did not have the quantity of cheap steel necessary for this, so they ensured only compressive loads on their concrete, so it lasted about as long as you'd expect a random rock subject to only compressive loads in a field to last.