Well the answer to that is no time soon. I am a loyal Fox News fan and never watch CNN.
But this is going to change this fall when CNN goes HD and Fox News, as far as I can tell, sticks to their 70's technology and continues to give us plain old vanilla SD. I can't help it. I just like watching HD much better than standard def TV. In LA the local news is pretty much HD these days and it very high quality. I think Fox had better wake up and get going with their HD programming or they will continue to see the ratings slide that has, I believe, already started. Part of the reason for that slide is their foolish NO HD policy. It is going to bite them.
Anyway, you're right: they had better. :)