I'd like tackle the inquiry of how much information responsible adults are, or should be expected to know in order to be informed citizens. I hope my interpretation of the question is accurate; if not, I apologize profusely.
I might first suggest that evaluating such a threshold from a purely quantitative perspective (volume or amount of information, i.e. how much) is impossible. But, as Lippmann suggests (and documents having experienced), modern society mandates that we have a knowledge base developed at least enough to function within individual circumstances; we're all busy, and besides, “Modern society is not visible to anybody, nor intelligible continuously and as a whole. One section is visible to another section; one series of acts is intelligible to this group or another to that” (Lippmann 39). Conflating quantitative and qualitative measures of "enough knowledge" might be appropriate here; the plumber presumably doesn't have, nor do they need, the same "amount" (or type) of knowledge about interest, credit, and private equity that the accountant does. Similarly, the accountant doesn't need an intricate understanding of pipe laying or water pressure systems.
By the way, plumbers make remarkably similar (if not more) amounts of money to accountants, but one occupation requires "more knowledge" in the form of a four-year degree. Take THAT, meritocracy!
I digress. To provide a blunt answer, I'd say that "knowing enough" to distinguish issues in the public forum which affect one's individual circumstance from those which don't, along with the ability to think critically about them, is adequate. Lippmann makes a distinction between private and public opinions: "A private opinion may be quite complicated, and may issue in quite complicated actions, in a whole train of subsidiary opinions, as when a man decides to build a house and then makes a hundred judgments as to how it shall be built," whereas "a public opinion has no such immediate responsibility or continuous result" (Lippmann 40). The result, "whether an idiot or genius has voted," is "a pencil mark on a piece of paper" (Lippmann 40).
Given this, while still cumbersome, the reasonable parameters of what one could be expected to know narrows considerably. Unless one writes professionally about popular culture, my guess is that Nicki Manaj is somewhat irrelevant in scope compared to, say, health care reform (which should really be called health insuance reform, anyway). It's not particularly laborious to form an opinion about the latest prefabricated pop icon; they're literally designed to be as consumable, marketable, and accessible as possible. Health insurance, on the other hand, is nowhere near as simple in its mechanics or underpinnings, but as Lippmann explains, the very few who guide public opinion on the matter, in any direction, would have you believe that it's an up or down scenario; agree or disagree, not necessarily with reform's long list of specificities, but with the entire idea.
Put simply, Nicki Manaj is inconsequential to one's rights and livelihood in public space. An individual's ability to access quality health care at reasonable cost is not. Yes, it's complicated, and requires broad understandings of many subjects (economics, history, civil liberties, political science; the list is endless). However, in this instance, a citizen's ability to follow the discourse surrounding the reform of its access could have a profound affect on whether or not one supports it, and the scope of its impact is undeniable: every American citizen. Such an opinion should not, nay, must not be based on a straightforward process that those who legislate would have us adopt. It could potentially betray the ideals of a free and just society, and is ultimately to no one's benefit but theirs. Apply this to other hot-button civic issues (gay marriage, abortion, gun control, take your pick), and I believe a similar conclusion is reached within respective nuances.
Certainly, the majority is susceptible regardless, but the individual doesn't have to be complacent. What I'm providing is admittedly normative in its cultural assumptions, but my answer is thus: it should be reasonable to expect an American adult to know enough about issues of personal and public import to distinguish, understand, and think critically about them. How these individual qualifiers are defined, I think, would be an interesting discussion.