Ours reads them within 48 hours, I think, for screening. May be shorter for diagnostic mammograms. They send a letter within 5 days, so they say to give it 10 days (with weekends and mail delivery) before calling. They also now have an option for someone to call you within 3 days (I think) either way. They will always call if there is any issue.
Sometimes if there is something obvious or you have had suspicious areas on previous ones, they will have someone look right away and send you to ultrasound right then.
Mama to three boys ('03, '05, '07)