Diagnostic. Mammogram should be interpreted by the radiologist while you wait.
Mammo results. The diagnostic mammogram is evaluated by the radiologist while you are there, so in most cases results should be available the same day. If you don't get them then, call your doctor's office in a day or two and inquire.
Not long. You should get results that day from your radiologist. The final report is usually available same day or the next.