It seems like Google has published a security update for Nexus devices. https://source.android.com/security/bulletin/2016-01-01.html
I checked system updates and it says that my phone is up to date but the recent update is from december. How long it takes until the new update shows up for download? My phone is a Nexus 5X.
I would guess it's up to your carrier to decide when to push the new update. This explains why some devices, though exactly the same get updates on different dates.
Try googling your carrier and the update and see if any news shows up as to when it might drop.