Frequently. Depending on the correction to the teeth, many orthodontists fit you with a retainer you must wear in order for the teeth not to go back to their original position. Sometimes, mostly in the lower teeth, the retainer might be placed permanently.
Long-term. "retention is a life-time commitment to periodic wear of retention devices." age changes us - that includes your teeth. Only way to prevent change is regular periodic monitoring and retention.
Retainers. Yes. I still wear my retainers. My first patient from over 10 years ago still wears her retainers and her teeth still look amazing.