Is a breast lump that hasn't grown in over 5 years unlikely to be cancer?

It is very: unlikely to be cancer. However, some cancers are indolent and very slow-growing too, so even though it seems stable, it is best to have your doctor check it out to be sure. Mammogram and/or ultrasound may be useful.

Biopsy: is the only definitive answer-never ignore and always get mamogram/ultrasound

Less likely: It is true that the longer a lump is "stable" , or not changing, the less likely to be malignant ( cancer). Many lumps can be safely followed with Ultrasound or mammogram. But ,as with anything, only Tissue biopsy can PROVE that something is benign