Next week seems a good bet. I could also see next week being the Nighthaunt vs Daughters box.
There's A LOT of Eldar stuff to come out, probably too much for it to fill one-week and the 19th is the Black Library Celebration, so I doubt they'd want to split it across three weeks. Plus they'd presumably want the FOMO box to sit for a while and releasing kits separately within a week might take away from that.
If it's not next week then I wouldn't be shocked if we don't see it until March. It feels like we only had the announcement/roadmap due to the negative reaction of LVO, which may hint at it being much further out.