How long could someone live with heart cancer?

Cardiac cancer. Carcinoma involving the heart is generally not survived for long. Malignant pericardial effusion or around the heart usually has survival under 6months. Cancers actually growing in the heart usually at least that short, it isn't a good situation.
Briefly. Primary rhabdomyosarcoma is difficult to control. Metastatic cancers from lung, breast, melanoma also are bad actors. Measure in months not years.