I don't use nag bags, but I do use something similar, it's called a busyhorse bag.
During winter I feed two times a day, usually all the horse can eat, but I always had a hard time adjusting the amount so that there wasn't any waste. With the busy bag I ended up feeding less because the hay stayed in the bag and off the ground. Another great thing is that insead of gobbling up her food immediately, she picked at it continuously throughout the day, keeping her occupied and out of trouble. The net I bought has 2" squares, and I would say that for 2 flakes of hay it takes her about 6 hours to get through.
"Horses donít have hard mouths, they have hard, stiff bodies. The softer you can get the horse through his body, the softer he will be in your hands." Clinton Anderson