I tend to divide villains into 3 categories based on how many sessions I intend them to last; session(1 session), arch(3-5 sessions), and campaign(most/all of the campaign). Obviously each type has its own merits, but what works best? How long should you keep a villain around for best effect?
Sometimes, you should take an arch... and reveal them to be a campaign.
If you find that a particular villain worked well (you like how it worked, the players enjoyed it and engaged with it), see if you can bring them back as a blindside as it makes a hail merry through what seemed like the main villain.
High ranked villains tend to be known to be complacent, while lower ranked ones tend to be hungry for power. It is nice for a betrayal. Have godhood stolen from the dragon king by his kobold steward that had been arranging all the details.