Thanks for your reply.
It might depend on the visa office though.
For example, most of CEC inland applications have a pretty consistent timeline. However, there is very little information on when the background check went from "not needed" to "in progress" for the second time.
I think, it would be helpful if we could figure out if it is just a random number of days that depends purely on each individual case or there is some pattern and it usually takes three days here and one month there.