Mana is expended when spells are cast, similarly to using your stamina or endurance to sprint. If you run too much or really fast, you're going to tire out quickly. Think of magic as the same. You'll become tired from abusive spellcasting in much the same way as fighting or doing other physically extreme activities, and you'll feel fine after resting just like anyone else. People with more mana will obviously be able to handle more use of magic.

This won't be a quantifiable mechanic in the roleplay, but really acting the condition of your character is encouraged.

Because of the wild and powerful nature of mana, spellcasting would be on average more strenuous than traditional martial fighting, but in exchange, magic is as versatile as your imagination.