It would be best to buy the feed more often so it's fresher. If the mill date is three to five weeks out, and you want it to last another eight weeks, that's a bit too long, IMO.
Chickens can, on average, eat 2.5 to 3.5 pounds of feed per pound of gain (body weight) so, depending on breed and conditions, 100 chicks to ?two pounds? body weight at eight weeks of age comes to about 450+ pounds of feed. Guessing here! Storing it safely is another issue! I have three large metal garbage cans for feed, and leaving the feed in bags, can store 300 pounds at once. I never buy that much because I wouldn't use it up fast enough.
Mary