How many lies = discredited?

You may have noticed that even though the Smear Boat Liars for Bush have been caught in numerous lies, the media keep trotting them out to spout still more falsehoods.

Josh Marshall raises a good question:

....what exactly is the statute of limitations on these guys? How many times do they have to get caught making false claims, unsubstantiated assertions or putting forward witnesses who weren't there, before they cease to have any credibility and get treated as such in the media?

At the moment the standard seems to be, "Okay, on your first nineteen claims, it seems like you were lying to us, but send along number twenty and we'll run that one up the flag pole too."

I'd sure like to see some editor or news director address that question.