Breast feeding lasts considerably longer. It's actually not as beneficial for the child to nurse less than about a year and a half to 2 years. The upper limit seems to be around 7 years for benefits.
Check out Katherine Dettwyler has a good essay on it, if you're interested. http://www.llli.org/ba/aug94.html
Wouldn't that account for lack of PMS "complaints"? Since not every woman experiences troublesome or severe PMS anyway. Out of the few who do experience moderate to severe PMS, they breastfeed for long periods thus avoiding PMS.