Your relative pitch gets decent within a few weeks of practice (<10h).
Getting it always right takes longer, and so do "weird" intervals, but the basic intervals are not that hard. Also, basic knowledge of song structure, phrases, and chords helps with relative pitch, because it narrows down the set of choices in many circumstances.
After all, the most useful exercise of relative pitch is "what's that melody", not "what's that random interval".
As for precision of relative pitch: After a year of weekly training, most people are able to tune a guitar to within 2-3 cents. Which is, granted, a very specific form of relative pitch, but it shows you can get pretty good precision at that. It probably extends further. (IDK, I have seen no examples of people explicitly practicing microtonal relative pitch outside of "tune an instrument")
Getting it always right takes longer, and so do "weird" intervals, but the basic intervals are not that hard. Also, basic knowledge of song structure, phrases, and chords helps with relative pitch, because it narrows down the set of choices in many circumstances.
After all, the most useful exercise of relative pitch is "what's that melody", not "what's that random interval".
As for precision of relative pitch: After a year of weekly training, most people are able to tune a guitar to within 2-3 cents. Which is, granted, a very specific form of relative pitch, but it shows you can get pretty good precision at that. It probably extends further. (IDK, I have seen no examples of people explicitly practicing microtonal relative pitch outside of "tune an instrument")