Some of my users are on Pixel devices and will get Android 9.0 the day it is released, while in best case, the source code only drops a week later. So assuming it takes me 10 minutes to fix all bugs, my app might crash for a week.
Realistically, my apps would be at least a month unusable.
By the way you can get a custom rom from here: https://forum.xda-developers.com/nexus-6p/development