Google start its monthly security update since November 2015. November Security Update here. December Security Update here.
The Android version is still 6.0.1. The date of Android security patch level was changed. Build number was changed when update too.
Before January update, the Nexus 7 information is shown as below.
The update notice looks like only 2.2MB
This software update will improve the security of your Nexus 7 with the January 2016 patches. Update size:2.2MB.
After updating, Nexus 7 information changed to below:
Based on the time on screenshots above, it took me about twenty minutes to do the whole update and optimization for existing apps.