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.
Yesterday we received pre-monthly bonus and this makes me change my thought that monthly bonus will be released before 15th. Usually monthly bonus come after 1 up to 2 weeks from the pre-monthly. It seems that the monthly bonus will come after 15th, perhaps after 17th but for sure it is just a speculation.