On a scale of centuries ...
The second leg of the race is on a scale of several centuries. The concrete structures will crack and degrade, but the radioactivity will still be contained in the packages. The actinides will have become the dominant elements. In 500 years the activity of our sample package, which was 1000 after 5 years, is now only 4.3 (NB: the timescales give for the degradation of the structures are only given as an example).
IN2P3/CEA