Based on what little information there is on swgohevents.com, I would guess 20-30 days, as the most recent return was 90 days after the previous, and we are 69 days since the most recent return.
https://swgohevents.com/event/dark-times
Shouldn't this been the one where it is here to stay permanently? Since it's will have been been a year? Pesky language tenses... https://www.youtube.com/watch?v=Dcu6GEti_94&ab_channel=BeyoTheQ