Honestly, it depends on whichever level of tier spells you used. Each tier level required different mp to use up. Barring the use of Metamagic Enhancements, a 10th tier spell would be worth 10 mp per use. That means they can used it at most ten times. Tier 9 = 9 mp, Tier 8 = 8 mp, Tier 7 = 7 mp, Tier 6 = 6 mp, and the pattern continues to trend that way as follow. Lower tier spell requires lesser mp to use while higher tier spells required more mp. You can use lower tier spells more times than the higher tier spells based on the amount of mp required per tier level.