Its up to 6 lbs a day until they are I think 20 weeks old. AFter that reduce to 2 kilos a day. Otherwise they do get fat. Personally although we feed veggies, I would certainly feed at the 1lb per day per month of age ratio until they are nearly at slaughter weight or at least 4 months, otherwise you will not get well grown pigs. (Thats my experience/understanding).
At the end of the day its the eye of the master etc.