Don't run your Mash dry?

[jon1163]

Making pure corn whiskey, a professional guide for amateur and micro distillers by Ian smiley.

My buddy was telling me about this part of the book that talks about distilling no more than 96 hours after you pitch yeast. I read the book but it doesn't say why. Does anyone know anything about this? Here's the quote:

"As a general rule, a fermenting mash should be distilled as soon as the vigorous primary fermentation stops, or slows down to a spurious bubbling, regardless of how complete the fermentation is. So, unless the match is still fermenting vigorously it should be distilled after no more than 96 hours after adding the yeast".