Is Backlog Refinement a Time-Boxed event?

Can anyone help to understand
" Refinement usually consumes no more than 10% of the capacity of the Development Team."

Here is my understanding :

Estimated Capacity of a team = Committed dev team members x( Effective Hours x No.of Sprint Days)

Hypothetically team members is = 4 (Fully committed)
Effective hours/per person = 6 Hrs
No. Of Sprint Days = 20 days(1 month sprint)

So the Capacity = 4*(6*20) = 480 man hours.
10% of 480 is = 48 hours which can be spent for the product backlog refinement.

Is it Time-Boxed?
do we need to convert this as a time-boxed of 2 days or 48 hrs for a sprint of one month?
in this case the time box varies based on the effective hours invested based on person's focusing factor.

Tnx