If I remember right (at work, posting on my phone) purpose lasts for 10secs and returns a % (10%???) of your mana for each melée hit that lands while it's active. if you time it right you can get about half a bar of mana from it, but if you have to move etc and only hit once or twice it doesn't do as much for you. Abilities still cost normal mana while it's active as well, so I try to use my single target spam while it's up instead of even justice spam.