I've been there as late as 11pm after the 9pm illuminations (about the same 90 minutes or so) and there was no sign of anyone ushering folks out, and still a few folks lingering behind me. ; I'd imagine you have in the vicinity of 2 hours to empty the parks of the vast majority of people, where they decide it's time to send castmembers through to sweep out the last stragglers.
I've been in AK nearly 2 hours after close...not even leaving the Dawa bar area in Harambe until 1 1/2 hours after...and slow-strolled out. ; The park was basically empty through the Africa section, but when I got to the Discovery area, there were still more people than I expected, and Island Mercantile was still open!