Which dental conditions require active and prolonged treatment?

Perio, caries: The two most likely are periodontal disease and caries or cavities. Gum disease (periodontal disease) is usually something you don't cure but manage for a lfetime. If you get lots of cavities that requires multiple prolonged treatment, root canals often follow, crowns, bridges, extractions etc....

Perio Disease & Orth: Periodontal disease (gum disease) and orthodontics will usually require the longest term care. Ortho has an end-point though; whereas periodontal disease often times does not. Periodontal disease is something that patients will have to deal with and treat for life, because it damages the underlying bone that supports the teeth, this never "heals", but is managed and arrested.