I agree with Mike, a 1' blood - though they can take larger meals for their length than msot snakes becasue of their girth - shoudln't be taking large rats. The general rule is to feed prey roughly the same size as the largest part of your snakes girth. At 12" or 18" their girth won't be that huge. Their meal would weigh more than they would
I am feeding my 3' borneo large rats (and I just upped him to large rats recently). As for growth rates, they are like people. They all have their own rate of growth, though it can be influenced to some degree, it is still unique to each individual. Hope this helped