My mother 61 was operated for breast conservation surgery followed by chemo and radiation. How often should she get repeat mammograms?

Annually. Often one or more mammograms are done on a 6 month interval on the affected/operated side until one is satisfied that a stable scar has been established. Beyond that, annual mammography remains the interval of standard on both the affected and non-affected side.
Check with your Dr. Many doctors and institutions have their own protocol for following patients (survivorship). In general, patients are followed with imaging every 6 months for 2-3 years and then yearly. It will depend on the specifics of the diagnosis and breast density. I strongly recommend you meet with your surgeon, oncologist or navigator to find out the schedule for follow up.
Yearly. Initially, mammogram every 6 months for two years, then yearly. Yearly MRI breast is also recommended.