Are u talking about elo tempa ban list or standard ban list ?
I'm pretty sure it all depends on how powerful and overused certain clans are, because that's almost always a sign of imbalance. Right now there don't seem to be clans that are OP(tho' I wouldn't mind if Mona would rot in hell) so the staff don't really have reason to change the banlist.
They are probably gonna spend more time making weak clans more playable and creating replacements for banned cards rather then nerf the more powerful ones or release the beasts.
I'm still waiting for a 3* with 8 for Freaks, UR :
There is not set date or time when the staff update the staff bans. They just do it when they think they need to.
There isn't really a set time to ban,but the norm has been every 5 or so months,so they're like informal versions of seasons.
So i would say,every pre-season there's a new ban-list