Women above the age of 50 usually have breast tissues that are less dense, making abnormal tissues easier to detect through a mammogram. If no abnormalities are detected, the chances of a cancer developing in between screens (two years) will be low (though still possible).
For women between the ages of 40 to 49, the breast tissues are denser and a mammogram may not be able to detect abnormal tissues as well. Therefore, women in this age range are recommended to screen more frequently, so any changes in the breast tissues in between screens (one year) can be better detected.
You should go for a mammogram once a year if you are aged 40-49 years old and once every two years if you are aged 50 years and above. As such, you should wait till you are due before going for your re-screen.