We don't know. No specific date is released, and based on past experiences with Smogon's tier system, there probably won't be one either. It'll be done when it's done. Give them some time to test them out, ORAS have only existed for a couple of weeks.
Also, when the tiers are released, don't expect all of them to be permanent. It takes time for the game to stabilise and for people to find what works and what doesn't.
