So, a couple things.
First, you can always check swgoh events for background information on questions like this:
https://swgohevents.com/event/duel-of-the-fates
That said, they keep track of how long it has been since the last one, and the average difference in time between events, and then make predictions. But CG can always just change their minds about how often these events will happen. If that happens and **if** CG has made an announcement about the change, always prioritize a CG announcement over a SWGOHEvents prediction.
SWGOHE says it's been 260+ days. I guess I hadn't realized it had been that long, but I trust them. There's clearly been a decision at CG to push people to relic 9 toons for new events, then make the events temporary, so you never get back as much as you invested to be eligible in the first place. (Or at least it takes so many years, it doesn't lend itself to a cost/benefit calculation in that way.)
My personal opinion is that this is a bit of a thank you to people who buy/rush the toons. It compensates you a bit for the relic mats invested, but it's not there to be an ongoing source of profit.
Will it come back? Sure. I think there's never going to be a day when we can say it's gone forever until the game itself shuts down. But in this case, since I haven't heard about an announcement and the SWGOHE prediction is based on data that isn't holding true this time around, I'd assume that once or twice a year is the new normal for Duel of the Fates.
If you have to guess, you could guess something special like May 4th-related game stuff, but it's also possible that it will just be random.