(roughly 5 days worth of milk - Costco does have its benefits)
As part of SS, Mark Rippetoe recommends drinking one gallon of milk a day on top of a solid diet. "heavier is eventually necessary if stronger is to occur" The milk aids in increasing the amount of calories consumed in a day. There are other ways of gaining weight but as a student I opted GOMAD. "Milk works because it is easy, it is available, it doesn't need any preparation, and it has all the components necessary for growing mamals." (Mark Rippetoe, 2nd ed. Starting Strength) There is more to it but that is the simple version.
Ben actually emailed Robb Wolf about this (check out his blog on the right hand side...he is truly an expert on nutrition for athletes) and he gave it the green light to try for 2 months. I trust Robb more than any medical doctor or someone who has a PhD in nutrition. Sill not conviced? Here is what Rip said to someone asking about GOMAD, "It is because over thirty years of direct observation has demonstrated to me that when trainees drink one gallon of milk added to their regular diet and train in a progressive linear fashion, they gain significant muscular bodyweight, and those that do not drink their milk, even in the presence of progressive linear training, fail to do this. They also fail to continue progressive linear training for the same length of time, because this is facilitated by the steady weight gain. I understand that you're asking me if I have controlled for other factors such as failure to do the program correctly, and the answer is yes, of course I have, because I am not a complete idiot. Those that will not do the program are not being considered when I make these remarks, because that would be too ****ing obvious a hole in my analysis. The difference in the milk drinkers is that THEY GET BIGGER THAN THE ONES WHO WON'T DRINK THE ****ING MILK. Please tell me that you understand this now."