Until the baby is done. It isn't the length of time, but the quality of the feeding that counts. My son and I both have a bowl of Cocoa Puffs every morning, the same amount, in the same size bowl. It takes him like 20 minutes to finish off the bowl, whereas I inhale it in about 45 seconds. It's the same bowl of Cocoa Puffs.
There are many extraordinary things about breastmilk. Chief among them is that the composition of the milk changes from the first feeding in the morning to the last feeding at night and from the beginning of each feeding to the end.
At the start of each feeding, the first milk the baby gets is called the foremilk which is designed to quench thirst. It is lower in calories and high in lactose. Lactose is a very important sugar that contributes to human brain development, helps absorb iron and calcium and promotes the growth of a healthy gut.
At the end of each feeding, the milk becomes higher in fat. This milk is called the hindmilk, and is higher in calories because of its fat content. The more hindmilk the baby gets, the longer the baby should go in between feedings. And nighttime feedings have more fatpromoting longer periods of sleep. You'll know when the baby gets the hindmilk--your child will come off the breast looking a little funny, totally zonked. I refer to it as the "milk buzz."
If we feed the baby, as many women are told "15 minutes on a side" we artificially limit the time the baby gets on the breast. Mom gets sore nipples because she has to break an often strong suck and the baby gets lots of foremilk and not a lot of hindmilk. Too much foremilk gets the baby lots of lactose, to the point where there isn't enough of the enzyme to break it all down, and the baby starts to show signs of lactose overload, like gas and really frequent watery stools. Plus, since the foremilk is lower in calories, the baby is eating all the time. Inevitably, mom feels like her milk isn't "good enough" or she starts examining her diet for the cause of the gas.
All we have to do is let the baby decide when they are done. The baby should get soup, salad, meat, potatoes, and dessert on the first side and an after dinner drink on the second, if they want it. Just begin the next feeding on the opposite side.