How commonly are liver lesions benign?

It depends. Benign liver lesions are found in routine imaging studies in more than 50% of adults. Even in adults with cancer, only 10% of liver lesions are cancerous. The characteristics of the lesion on the imaging study, the type of imaging study, and the size and number of lesions all play a part in determining the chance a lesion is benign. Radiologists are trained to decide when more tests are needed.
Often. It depends on the age of the patient and the presence or absence of associated illnesses that would put you at increased risk for malignancy. Patients with underlying liver disease are at highest risk for developing malignancy.