Virtually endlessly. What they're talking about is, AFAIK, the actual original (not actually original, but NES) Tetris. It was meant to be infinite, but at some point the numbers get too big to store, and the programming starts breaking down. Some games might be able to keep going indefinitely, just resetting/looping some numbers, and in modern games it might take years, centuries, or even universal lifetimes to reach that point, but almost all "infinite" games will break down at some point.