Feeding long enough

Rachel • Wed, Oct 21 • I have 2 amazingly beautiful boys! One turned 3 in December, and his little brother was born October 7th! 💙💙

How do you know they have ate long enough to empty you enough that they are receiving the fatty "foremilk"? My son varies anywhere from 15-40 minutes on one side side (usuall shorter in middle of the night). And my boob doesn't feel full afterwords.