By the basic rules, they can not have any supernatural abilities unless they channel a god. That means you need to spend Prime _every time_ a hero does something supernatural.
Since Prime is not like mana in computer games, i.e. there is no "spend twice as much for double the effect" rule, you can make your hero longlived by simply telling a story about how he reached old age and expending one Prime. Then if you want to re-use him in a later story, still older, you need to spend Prime again to make him still alive beyond his natural lifespan.