ALLOW_SNAPSHOT_ISOLATION Duration

Question

I would like to enable ALLOW_SNAPSHOT_ISOLATION on a production database that has ~400 million total records. I understand that 14 bytes will need to be added to each record.

If I set ALLOW_SNAPSHOT_ISOLATION will it block for a period of time proportional to the record count, or will the data be updated asynchronously?

Primarily, I want to be sure that my database will not be out of service for hours when this setting is enabled.