How to specify vbvBufferSize

The NVIDIA Encoder PDF says for “recording/archiving” use VBV buffers size of 4 seconds.

The comment for vbvBuffeSize says “Specifies the VBV(HRD) buffer size. in bits.”

What size is “4 seconds” then? Is this based on the encoding rate? So 12mbit/s would mean 48,000,000 for 4 seconds?