How frequently is cancer responsive to chemo/radiation?

Very!: Response rates vary quite a bit depending on the type of cancer, it's stage, and it's unique molecular features.

Often: Most cancers are radiosensitive enough that local therapy will slow them down ("spot welding" for bone pain, for example.) some cancers are exquisitely radiosensitive -- and some won't even come back. There are many chemotherapy protocols that i would refuse, others that any reasonable person would accept. An ethical physician will talk straight with you. Good luck.