I have read 1lb/rat/month. Of course large boys will eat more and small girls will eat less. That is how much my rats eat and 3 of the 4 are big squishy manrats. 1 is a tiny manrat but is hairless so possibly eats about the same as everyone else due to a higher metabolism.
It can go bad 6 months after the milling date. Keep frozen to preserve for longer (I think it last a year frozen possibly?)