Please tell me how long it takes for to get mammogram results back?

Mammo results. The legal requirement of the Mammography Quality Standards Act(MQSA) is that you receive a results letter within 30 days. Most often patients will hear from their doctor much sooner. If you are very worried, call your doctors office for results. Typically the report should be available within a couple of days.
24 hours. This may vary quite a bit depending upon where you get your mammogram. There are many centers that offer immediate "reads" of mammograms if you are willing to come in during normal business hours. I think a better question is how long are you willing to wait to get a result? If you want immediate or fast (<24 hr) results, it is best to shop around. Be sure to choose an fda-accredited facility.
Varies. Most institutions mail you a letter in 1-2 weeks if the results are normal. If it is abnormal, you will likely get a call from the radiology department or your doctor within 1-5 days. There are some places where you pay completely out of pocket/no insurance accepted that offer same day results.