aura duration

They have no duration limit. The system here, is that it reserves a porcentage or a fixed amount of your mana, and in exchange, the auras are permanent. So instead of paying 20 mana to have X aura X amount of time, you pay a lot more mana, but get permanent auras. This proved to be a really neat system, wich I personally LOVE.
Extra tip: If you start using auras, you should be aware that each time you level them up, they require substantialy more mana, so there are some aura gems you may wanna leave at a certain level and stop hitting the lvl up on them.