Canon: Specific & Vague
When looking at the history of D&D lore on this issue, we have to confront the fact that the majority of sources are nonspecific about how far in the past the uprising took place, apart from noting that the illithid empire was "ancient," as was the uprising.
There is one specific reference, though the number it gives doesn't square with other suggestions that the empire's existence was "in a time before most of the prime worlds known today were born." That number is found in Lords of Madness, and is given as two thousand years. That range is fairly inadequate - among other things, githyanki have a lich-queen who claims to be the 157th in her line, twice the lifespan of humans, and a current residence that does not even experience time, such that caretakers of githyanki eggs and offspring need to truck them to the Prime to mature properly, periodically. Given what we know about dragon lifespans, two thousand years also feels fairly inadequate for a long and storied allegiance.
Twenty thousand years would be more reasonable - a vast span of time, enough for the githyanki and githzerai to establish well-known cultures relevant on the Planes, enough for the illithids to have rebuilt some of their strength and made several new attempts to conquer existence. The ancient empire was vast and terrible enough to startle the fiends into pausing the Blood War itself; was capable of engineering an artificial discworld with the diameter of a planetary orbit in a mere thousand years; and its scattered splinters were able to establish themselves on worlds across the Prime, worlds like Toril and Oerth, where even in small numbers they became forces to be feared.
The Nature of the Rebellion
The duergar uprising isn't really comparable to the gith uprising, scope notwithstanding. The most salient quality of the gith uprising is that it happened because the illithids were no longer content to just use their forerunner slave stock as mutable thralls - a threat to the empire from a species known as the voor resulted in the mind flayers developing new mutagenic methods to weaponize their favored servants against this new adversary. The tumerogenesis tank, a kind of mutation pod, was used on forerunners to trigger physical and spiritual enhancements, as well as psionic capacity for the sake of fighting the voor.
Whatever modifications were made to the victims of the tumerogenesis tanks would be considerably more substantial than the present-day differences between the gith races and humans. These changes, made at a genetic level, created ideal warriors to combat the alien menace, but the mind flayers wisely recognized the threat that this new super-soldier breed posed to them, and moved to eradicate the whole stock once the conflict with the voor was resolved.
Obviously, they failed.
Some few were able to conceal themselves and move among the forerunner population, mating and passing down some of their changed genes. Gith was an heir of these bloodlines, these strange transformative mutations providing traits that would arm them to face the illithids - not one-to-one, perhaps, but given both psionic durability and sheer numbers, the mind flayers were wholly unprepared for what came next. The gith toppled their masters without history, religion, territory, anything on their side - except the weapons the mind flayers inadvertently gave them. As for the battlefield, the duergar fought in the Underdark; the gith fought on the artificial world of Penumbra; on the forerunner homeworld of Pharagos; on nautiloid ships across wildspace in countless crystal spheres.
Anyway, I hope this has been at least interesting; hopefully helpful.