There’s no real pattern, however. People like to try to find one, but there isn’t.
Nightborne: Introduced in 7.0, made playable in 7.3.5
Highmountain: Introduced in 7.0, made playable in 7.3.5
Lightforged: Introduced in 7.3, made playable in 7.3.5
Void elves: Introduced and made playable in 7.3.5
Mag’har: TBC/WoD race, made playable in 8.0
Dark Iron: Vanilla race, made playable in 8.0
Zandalari: Vanilla race, revamped in 4.2, then again in 5.2, then AGAIN in 8.0, made playable in 8.1.5
Kul tirans: Introduced 8.0, made playable 8.1.5
So, let’s forget the races that were already established and just look at the new ones:
Nightborne: Introduced in 7.0, made playable in 7.3.5
Highmountain: Introduced in 7.0, made playable in 7.3.5
Lightforged: Introduced in 7.3, made playable in 7.3.5
Void elves: Introduced and made playable in 7.3.5
Kul tirans: Introduced 8.0, made playable 8.1.5
See? There’s no pattern. We can’t really consider Nightborne and Highmountain, as they were Legion races, and allied race system was introduced in BfA.
Lightforged were obviously made already with Allied Races in mind, thought. And they were made playable in the next minor patch.
Void elves too were introduced with allied races in mind. They became playable in the same patch.
The first pair of Allied races were older races that were available in 8.0.
Then the next pair are races related to BfA main continents, and will be available in next minor patch (8.1.5).
So, the only thing that is required for the next allied race patch (8.2.5, probably) is that the races have been introduced by then. It doesn’t matter if they are races from 8.0, 8.1, 8.2 or something older.
8.2 will introduce Mechagon and its storyline. It has the potential of containing all that junknomes need to become playable. So, it’s entirely possible that they could become available, 3-4 months later, in 8.2.5.