Mental Endurance

Mental Endurance is the overall amount of Magic a user to cast before they suffer increasing degrees of mental exhaustion. A user with high endurance will have the following benefits:

Increased capacity of Mental Reserve
The Mental Reserve is a limited resource used to restore Mental Energy during combat, and can only be restored by resting the mind or sleeping. Having a larger mental reserve means being able to cast more spells before becoming mentally exhausted. As mental exhaustion builds up to the limit, the user will find themselves less and less able to think, as their mind will become to befuddled. For example, a user who is running out of endurance may fail to properly cast a piercing Ice Spike, and instead, launches a pathetically small ice cube as a projectile. If pushed beyond their limit, this can lead to Mental Incapacitation.

Decreased penalty from Spell Combos
When spells are casted over and over again in a short period of time, especially if they're different spells, they can result in a stacking up of penalty to the required Mental Energy needed to cast the magic - which increases for as long as the combo is kept up. For this reason, it is much more efficient to for example, shoot two to three fireballs, and then then wait a second or two before shooting more. However, better mental endurance reduces the penalty, allowing magic users to, for example, fire six to seven fireballs before starting to incur this extra penalty.

Training
The regular way to train this is to cast magic until the Mental Energy is depleted, and waiting a few moments for it to recharge from the Mental Reserve until the mental reserve is completely depleted.

This can also be trained by non-magical methods, which is staying focused on something that requires high effort not only long periods of time, but also staying focused in spite of several distractions. Nothing is learnt if the user cannot stay focused, so the amount of distractions and intensity of the focus needs to be balanced to be just above their current limit. Trying to over-do it will not work.

Persisting through sleep deprivation, especially if mentally taxing tasks need to be done is an effective way to train this. In addition, certain digital games that require "marathon-ing" are good at training this. Drawing is something that also helps, as it requires staying on task for a prolonged period of time.

Examples
An example of a Freak with extreme mental endurance is Binary, who is able to continue using the few magical powers she has such as her signature Mortal Struggle as well as her Detect Lifesig. Her mental endurance is so large, that she had to spend 20 hours focusing her mind and body on keeping herself from getting hurt during a tortuously long struggle, allowing Katie enough time to save her from potential extreme pain if she had let herself go. She is the only freak to surpass Katie when it comes to mental endurance.

See Magic Stats for more stats that are inherent to a magic user's mind.