I've been wondering this for a long while. Is it ever confirmed or told just how far in the future Grovyle and I came from?
No, it is never revealed exactly how far. However, if you can manage to pinpoint the timein which the Special Episode In a Future of Darkness happens, you can find out; they came from the same time that Dialga started going berserk.
Source: I've played EoD once and EoS twice.
It is never said just how far into the future your character is from, but going off what Celebi said about her needing to use the Passage of Time for trips spanning generations, which she needs to use to send your character back in time, and since a generation is about 30, my best guess would be that your character is at least from 90 to 120 years into the future (since Celebi makes a point of saying 'trips spanning generations', it is definitely more than one or two generations into the past that you travel)
Idk hope this was helpful.