What kind of routine follow up should a person with harrington rods have over the lifespan?

None unless symptoms. Unless there is pain or other nerve symptoms, no follow-up is necessary.
Depends. If you are not have any issues or if the issues are stable and mild (like a backache that does not interfere with activities, lifestyle), no follow-up is needed. If you are having new or worsening neurological symptoms (radiating leg pain, numbness, tingling, weakness; trouble emptying the bladder), see a spinal specialist.