There is no fixed timeline. Going by the Mavic Pro, it was released at the end of Sept 2016 and by early November there were already third party apps so it was probably added to the SDK's at or before the same time as the release of the drone.
The Mavic Air, on the other hand, was released in late Jan 2017 but SDK support didn't show up until April.
Dji almost seems to be on the fence about the Mini, though, from what I've been able to glean. There's some speculation they're reluctant to release SDK support because they don't want to undercut sales of their more expensive drones too much by making the Mini capable of most of the same things. My guess, if that is the case, they're probably going to wait until they see how Mini sales affect the sales of their other drones before deciding. Which means it could be several months, if ever.