Both Legion did a tier 1.5 mini raid with Trial of Valor about 2 months after Legion release. BFA got Battle for Dazar'alor (full raid) 4 months after release and the Crucible of Storms mini-raid 7 months after release.
So if the last two expacs were any precedent, chances of at least a mini-raid, if not a whole raid, within 6 months are pretty solid.