Its available every fight. And its almost required on mages. Its just a silly mechanic of who can cheese who. Homogenized the game, because no matter the team comp, you are going to deal with some asshole always having his or her hourglass ready for every team fight.
Its not satisfying. I wish I could just ditch that feature out of the game.
And dont get me started on that fucking stopwatch rune. its 25 minutes into the game and Yas, out of no where, hourglasses and you realize the asshole took it and had it this whole time.