With AI War I intend to keep doing expansions for a long time, but that's because I don't plan to do sequels. So it will likely be an expansion every 8 to 10 months until... whenever people stop wanting them. I think the end volume is less important than the rate at which stuff like that comes out, personally, but I know that might vary. When too many expansions come out too fast, that's frustrating for me, too.
I haven't played Arkham Horror, but I really do love Descent, which is also by FFG (and in fact, I created a faster variant of Descent that is still pretty well liked). I have the fist three expansions to Descent, but I still haven't played all of the quests with them, and there are already at least a couple of more expansions since then that have been released. I'm glad to have the extra content, most specifically the new monster figures, but after a certain point it just loses me -- mainly, when they increased the rate at which expansions were coming out.
But, on the other hand, I don't begrudge them for that. If they can sell that many expansions, and the core Descent playerbase really wants that many, then I'm not going to gripe about it. What I have is complete enough, unlike in the sense of little DLC microtransactions. Those expansions are separate products, in a sense -- whereas missing cards in a CCG/TCG, or unbought DLC for a base game, generally means there is something "missing" from the initial product, or that's the sense it gives me.
Yah, so convoluted and contradictory, in many ways.