The ban must be announced within 14 days of the the card becoming legal in the format in paper OR the card becoming legal in the format online (i.e. ignoring any pre-release events).
Pre-bans don't count, the card has to spend some time being playable in the format before it's banned.
Alchemy-style nerfs are not bans.
The card must be banned from one of these formats: Standard, Alchemy, Explorer/Pioneer, Historic, Timeless, Modern, Legacy, Vintage, Pauper, Commander, Brawl, Standard Brawl. Bans that hit a subset of one of these formats (such as a card only being banned in Best of 1 Arena Standard) still count.
1,000
3.00Given that Nadu took over 2 months to be banned despite very obviously needing it, I doubt this will happen outside of a coincidence. (along the lines of, a set with an OP card is released less than 2 weeks before they release their regular ban announcement)
@Nightsquared This happened a couple years ago with Tibalt's Trickery in Modern. Not a high base rate but not nothing either.