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.