well can you. I'm very literal at times when someone says something metaphorical my first thought is picturing it literally. it took me years to figure out about 15 minutes didn't mean exactly 15 minutes and would get mad when it take 16 minutes.
i can't answer your main question about the S vs. N preference as i do not (yet) have sufficient knowledge of this, but:
the example you gave seems to be more about pragmatics than anything else - that is, it is about social context rather than the meaning behind the words themselves. there is nothing metaphorical about the phrase "fifteen minutes" in itself, but it might have a different meaning depending on the specific culture. in my culture being late is common, sometimes even in official settings, and punctuality is not among the most highly rated traits, so when someone says "in fifteen minutes", they are more than likely to take half an hour or longer. where i live, in lithuania, people tend to be somewhat more punctual, but still, when someone says "fifteen minutes", they are probably referring to a vague temporal interval that might be slightly more or slightly less than that. on the other hand, my cousin who has emigrated to germany would say that "fifteen minutes" over there would probably mean just that. or, if one were told to arrive at particular hour, for instance at 6p.m., one would have to be there at six sharp, whereas in russia it would be acceptable or even required to be a little late (so that "be there at six" would mean "be there at 18.15 or 18.30"). it seems to be much less about your individual cognitive preferences and much more about the social context you were brought up in.
there is also another type of literal-mindedness where one has difficulty understanding figures of speech (metaphor, simile, allegory, certain sorts of symbols etc.) they way they were intended. for instance, you would hear a poetic expression - such as, say, "the copper of the leaves is flowing quietly" or "the bells of the stars ring silently", if we take the work of my favorite poet sergei esenin - and you would think, how on earth are leaves like copper? or, how could stars ever be like bells and how it is possible for ringing to take place without any noise? you may be able to understand these expressions rationally and trace the semantic connections they were based on, but they would still make no sense or produce a feeling of distaste/discomfort. that would be more like genuine literal-mindedness.
i've been wondering about the same question, really. i can't say i am literal in the second sense. i have no trouble whatsoever understanding figurative speech, i love it and am a huge fan of poetry or poetry-in-prose (again the more figurative, the better). if anything, i often feel metaphor may be more apt than regular, straightforward language; this is particularly applicable to emotions, which i sometimes have severe difficulties identifying and defining. the words "sadness" or "joy" mean little to me, since each encompasses a vast variety of subtle emotional undertones which i might sense, but be incapable of describing, and, to be quite honest, there have been times where i couldn't tell whether i was experiencing one or the other. but expressions like "i feel i have turned into stone" or "everything is melting and too bright, the world is like a shimmering screen that is about to morph into something else" or "like a crust of ice crawling all over you, thickening" do tell me a lot because they directly relate to certain internal states. when i run out of "ordinary" words, i start speaking in metaphors in an attempt to convey what i am feeling.
on the other hand, i can be very bad with implied meanings. if someone' s speech is implicature-heavy and they drop hints without getting to the point, the likelihood is that i am not going to "get it". i usually ask people to tell me what they want from me outright, as this spares a lot of confusion and miscommunication on both sides. and i used to have difficulty understanding irony/sarcasm and humor as a child (i used to take everything seriously, which meant the most light-hearted comments left me insulted); these days, humor is fine, or at least slavic humor is, but i do occasionally miss irony, especially when the shift in voice tone is not that noticeable. that makes me wonder whether i might be S, too.
