5 minutes ,of course.Train well before go to the cavern(Learn how to use Squall'sGun-Blade,be sure that you can make a critical strike everytime you attack ).Then just run from any battle you encounter in the cavern.You'll have about 3,5 minutes to fight Ifrit.DON'T call Shiva(waste many time)!and don't use Magic,too(except "Cure"-only use when your hp very low (about 150-200)-),useless of time.Just "Attack"!Very quick action and you 'll finish Ifrit in time.That's for sure!