What is the risk of breast cancer when grandmother had mastectomy?

Maybe: First degree relatives such as mother or sister raise a person's risk for breast cancer. There is a measurement of breast cancer risk called gail risk model that you can check. http://www.cancer.gov/bcrisktool/.

Average: If your grandmother is the only family member with breast ca, your risk should be about the same as the average women who is your age, with the same menstrual, pregnancy history, etc. First order relatives (mother daughter, sister) increase your risk and multiple second order relatives may increase risk. As well.