But if this is just an online version (it is /fast, whatever that means), it might be a different story.
AFAIK /fast is an URL to read just the comics "fast" without spending bandwidth loading all the additional information presented in the landing page of the website.
Doonesbury has a 10 day lead time, which is still challenging for Trudeau to remain topical: http://www.doonesbury.com/strip/faqs/faq_cs.html
edit: My bad, it was the iPhone 4 prototype debacle! But he directly answers the question about how far he works ahead. http://www.dilbert.com/blog/entry/thatlost4gphone/
If he even has the ability to get one approved and published quickly, which I'd say is doubtful given that it's print, it's probably a nuclear option that he wouldn't use for having seen a few threads here.
In one of his books, he talked about this in greater detail. One of the examples he cited was a comic where someone made a joke about a "dead nun in a snow drift" - that just happened to run the same day as a bus crash that left a whole bunch of nuns dead.