I get asked for my authenticator code periodically.
Typically once a week.

Usually when I'm doing something that is atypical.
Or when I haven't logged in for a while.

All five accounts had to do that, after the Pally/DK's transfered to Magtheridon.

If I haven't been using my auction toon for a while, and then log in solo.
That will usually require an authenticator.

If I take a week off from the game.
I usually need the authenticator again.
I logged the trade toon in solo, needed the authenticator for that account.
And then logged the team in a bit later, and needed it for the other four accounts but not the recently authenticated account.

It's fairly random.
But it's also very infrequent.
This is from the same computer, with the same IP address, every time.