Given the current situation, we anticipate that it may take several months or more to add new Worlds.
So basically, months.

What we're waiting for right now is the expansion to die off or lose player base. That's literally the only way it can get better. However, despite the queues the expansion itself is amazing, WoW is in the dumpster, there's very little competition and we haven't even started on the raids yet. Even if things settle down, players will think "oh, it's playable now" and drift back. So, who knows. It could indeed easily take months.