Not quite sure that really works either. In the mirror world Sunset is no more than a highschool senior and the counterparts to the M6, and presumably Twilight inhuman form as well, are no less than sophomores. so the age gap should only be about two years, or maybe a bit more if we presume Sunset is just pretending to be young enough for highschool. Still, she appears to be getting progressively older at about the right rate in the pictures of her previous fall formal coronations.
Anyway, for the pictures to be accurate she'd have to have been in the mirror world for a minimum of 3 years, and since the portal only opens intervals of 30 moons (roughly 2.5 years), I'd be inclined to believe she'd been in mirror world for two such cycles, or roughly 5 years -- no more no less.
That's a possibility, but Luna said the portal would remain open for three days, and that's how long Twilight had once on the other side as well, though maybe time is only synchronized while the portal is open.
Still, an interesting thing to note is that the 2.5 years between openings could correlate nicely to the two full-seasons and one half-season of MLP preceding the movie. Meaning that Sunset might have temporarily snuck back into equestrian round about the time of the first episode, which is when she could have first formulated her plan to steel the newly revealed Element of Magic on her next trip. This also would fit with how in the picture of her first Fall Formal coronation Sunset looked genuinely happy (as it would have been before said visit back to Equestria), but in the next two pictures she looks decidedly more sinister.
If the original human Sunnset was a student at Cannterlot-High, that might create some rather unfortunate implication in regards to what pony Sunset did to her, since obviously they can't both be attending school at the same time.
If Sunset has a human counterpart, I'd be more inclined to believe that, like Twilight's and Spike's doubles, she lives in some other town.