Well, the first flashback chapter is numbered -108. It's been counting down from there (well, up, but you know what I mean. -107, -106, etc). That's a hundred chapters, or thereabout, before it hits zero. You release it on a weekly basis, that's two years.

Who knows, maybe he'll stop it before that and pick it up again later. I hope to god that he does. I mean, the flashback arc's good, but it's not so good that he should spend two years straight on it.