When you start getting Mammogram's done between the ages of 45 to 55 or before those ages you need to have a mammogram every year at minimum.
Once you get past age 55 and above you should have a mammogram every 1 to 2 years.
However if you want too you can continue to have mammograms every year if you want too but every 2 years is a good enough amount of time between mammograms for women above 55 years of age.