Roughly every hour. Instances (doing dungeons, raids, trials) makes it worse - the game crashes once per run guaranteed, which is bad if I'm tanking.
Hanging out in the open world seems to last 3-4 hours.

Sometimes this:
http://i.imgur.com/Kju9ETu.png

but usually "r6025 Pure Virtual Function Call - " and the rest is truncated.

I am positive it has nothing to do with my machine, or network. To check if it was the router, I plugged directly into the cable modem, and still got the error.