How long do they store the data for?
What this does not tell us is how long the metadata is stored for. 217TB sound like a lot of data but in the grand scheme of things it isn't. Banking systems hold nearly as much.
Do they slurp up a days worth of metadata from across the globe, process it and then keep the bits they are interested in?
Or do they just download the entire internet's browsing history and file it away?
My guess is somewhere between the two. As another poster mentioned, this system would have been a govt IT project. The spooks probably wanted the first option and got something nearer the second.