Hard lump on gum under tongue been there over two months dentist is keeping an eye on it hasnt changed in size or appearane is this a good sign?/

Yes- Torus?: If it is a mandibular torus, which is merely some excess bone, it is completely benign and nothing to be concerned with. In fact, there is no reason to even monitor it. Without actually personally examining you and seeing x-rays of the area, there is no way to know for sure that's what you have. Ask your dentist if that is what it is or does he\she suspect anything else.

Yes: Yes no change is good. But i wouldn't you want to know what it is. I suggest a biopsy.