Package the milk up in 2-4 ounce bags...until you figure out 'normal' for your wee one... and just keep offering it until he doesn't want any more. Some newborns can only take 2 or 3 ounces. Some take 8-12. Others take up to 20.
We were part of a nursing study, and my son was weighed (with super spiffy electronic baby scales that went down to some very very small numbers) before eating and after. At 3 weeks old the least he nursed was 19oz, and the most was 22 (in one sitting). Over the next 9 mo, he consistently ate apx 20oz per feeding. We were the outliers in the nursing study, with only 2 or 3 other babies eating as much as he did (out of several hundred). Most ate 6-9oz per feeding.
((And no. He didn't go for longer periods than other babies between feedings. Two hours later, he'd eat again.))
But for feeding on demand, you just keep giving them milk until they won't take anymore.