What percent of women in the U.S. and/or Canada have been on hormone
replacement therapy over the past few decades.  i.e. what percent in
each period?

I wasn't able to find the percentage of women per decade taking
hormone replacement. The best statistic I got is that 45% of women
born from 1897-1950 (which would include the decades in question) have
taken some form of hormone replacement.

The number of women today is anywhere from 20-45%, depending on the
type or combination of hormone.

Your sources:
http://www.nih.gov
http://www.stanford.edu/dept/news/report/news/july10/hormone.html
http://www.nytimes.com/2002/07/17/health/17HORM.html
http://newscenter.cancer.gov/pressreleases/estrogenplus.html

claudietta