No one knows, but maybe we can hope that it will come within the next 10 days (at max) as we got December monthly bonus in the beginning of the month so I believe we will get it before 15th.
That means even bigger monthly coupon since last 45 days wager will be accumulated for this bonus. Btw, 15 January is Saturday, usually no coupons are released during weekends. Probably 1 or 2 days before mentioned date.
Now it seems like, monthly bonus will be released before the 15th January. The users are eagerly waiting for the monthly bonus. See the recent tweets of Stake twitter account, all of the users are asking for the monthly bonus by making a reply. Maybe they will release it in next three days.