How long can someone live that's been diagnosed with stage 1 breast cancer on average (beyond 5 years)?

Breast cancer: That is a difficult question. It is certainly possible. With treatment the 5 year survival rate for Stage I cancer is very high. Without treatment it would depend on the type of cancer, how aggressive it is, age and health of the patient among other things. I have seen advanced stage patient live a lot longer than expected and early stage patients die earlier. I would advise treatment.