Yes, it can happen that no editor is interested in a specific category for quite some time, even for years. Its maybe not to common, but since the timeframe is totally unpredictable... There is no "regular" shedule of any kind in which editors visit categories, because that would not matchwith our concept. But one point to remember: At least at one point in history, at least one editor has been interested, otherwise the category would not exist. In most categories, even in the obscurest ones that I edited in, I have seen edits by several different other editors before me, so obviously over time even obscure topics attract editor activity.
Coming back to your initial question, let me ask back: Does it hurt to suggest a site? What does it "cost" you to suggest a site you think should be listed? In the time you were discussing here with me you could have easily suggested half a dozen sites I suppose. It's not consuming to much time, especially compared with reviewing the site whch in many cases takes considerably more time. And to think further, if you are suggesting a site to a a field that is so obscure that you imagine no editor will be interested in it for years, you might want to think about becoming an editor for that area yourself. See the linked resources in our "Become an editor" forum for more details.