If you mean the colossal ring that's got plant and animal life on it, well... it's relative. If you start walking exactly parallel with the edge of the ring, and you somehow get over all the terrain, you will NEVER reach the end of the ring. But that means it only goes on forever in the same sense as Earth going on forever. If you mean the storyline, though, yes. Time goes on forever, so Halo will go on forever.