Continuous Nursing as a Contraceptive...

Today I'm reading through "Why Zebras Don't Get Ulcers" by Robert M. Sapolsky and on page 132 he discusses how prolactin suppresses ovulation (it suppresses estrogen and progesterone) and how breast-feeding may be effective birth control if done in a continuous way.
The way to keep the prolactin circulating so that it will stop ovulation is to keep it in the bloodstream by breast-feeding the baby about 1-2 minutes about every 15 minutes around the clock.
The original affluent society which sits around with lots of free time and mostly talks, dances, sings and plays games is the tribal hunter-gatherer people. They only actually work maybe an hour a day. The tribal mothers nurse 1-2 minutes at a time and nurse about every 15 minutes and will nurse during the night. They naturally get pregnant every 3-4 years.
In America today, a mother will nurse a long time each time, maybe 30 minutes or more each time. She will nurse maybe 6 times a day. She will avoid night nursing. The tribal mother will nurse maybe 50 times or more a day. Nursing 6 times a day is not enough to stop ovulation.
So when the mother goes out to dinner without the baby, she may ovulate.
Sperm can live four days. Suction or manipulation of the nipples to stimulate nursing would keep the prolactin circulating.