Hip-Hop music is (from its very nature) minimal, and for the most part it has distinctly repetitive song structures. More often than not, a song's main focus is a looped sample that plays throughout its duration, with other components just providing subtle variations. This fact inevitably raises the question "how come it doesn't feel monotonous?" particularly when one considers Instrumental Hip-Hop.
Listening to J Dilla's 'Thought U Wuz Nice' or K Otix's 'World Renown', one can quickly realize that the whole song could be summed up pretty accurately in a 10-second snippet, and yet nobody seems to mind. On the contrary, people often feel that is not enough, making and enjoying 1-hour looped homework edits. Of course, this minimalistic approach is not found exclusively in Hip-Hop, and exists in many other genres such as Deep House, Chillout or even Minimal Avant-Garde.
But why do we find ourselves enjoying this seemingly mundane music, and why do we often seek for more?
Theodor W. Adorno
My quest for academic answers failed miserably, since when one searches for "loops" or "repetition" most books and articles rather focus on the brainwashing we endure on a daily basis by contemporary Pop Music. In more detail, there have been many published works which explain that if you listen a song over and over (regardless of how conscious you are while doing so), it eventually grows on you, no matter how strongly you attempt to resist it. [If you want to explore more about this topic I suggest you start with Adorno's controversial essay 'On Popular Music']
But despite being true that our brain's only criterion to 'get hooked' on a song is merely repetition, I don't think it's the decisive force at play here. Surely, the two are probably interrelated on a deeper level, but the fact that you enjoy a 5-second loop played over and over is not quite the same with the fact that you find yourself singing Gangnam Style in the shower.
In my opinion, the answer is how you listen to these kinds of songs, or how the songs themselves make you listen to them. If an instrumental song is shaped around a very short repeated loop, you can't help but step back and listen to it in a different manor than you would a lyrical, or a Pop one. The latter are designed to attract and maintain your attention, although when we talk about Hip-Hop Instrumentals, they seem to purposely have the opposite effect.
But this state is not to be confused with passive listening that you might experience in a shopping mall or a restaurant, as they are quite distinct both on how you perceive them, and how they were meant to be perceived. Subjectively, the experience is analogous to that of intentionally blurring your vision when looking at a landscape; you can still be concentrated (or at least aware) on what you are seeing, but you choose to look at the broader formation of colors rather than the details. So by this train of thought, it doesn't matter how long these songs last, whether is 3 minutes or 3 hours, the question is how long can you manage to maintain that state.
Of course this kind of minimal instrumental songs also allow you to completely phase out (like you easily would on a blurred vision), and use them either for studying or background. But it is almost impossible to purely focus on them and maintain that focus, as they instantly slip back to their preferred attention levels. Even if you manage to extensively focus on them, you will be missing the point, and probably get tired rather quickly.
In a nutshell, the beauty of Instrumental Hip-Hop (or any minimal instrumental music for that matter) is its very simplicity, which forces you to step back and perceive the piece as a whole, rather than be blinded by attention grabbing elements. It's definitely easy to completely disregard it, and that is OK, but it isn't merely background or studying music.