Elsa
Elsa's magical endurance appears functionally limitless under normal circumstances. Maintaining a kingdom-wide winter for several days produced no visible fatigue, whilst the construction and indefinite sustenance of multiple sentient constructs proceeds without apparent magical drain.
Her abilities appear to draw upon either innate reserves of extraordinary depth or some external elemental source, as no documented instance of magical exhaustion has been observed. The sustained cryogenic output necessary for her architectural achievements alone would exhaust conventional magical practitioners within minutes.
Harry Potter
Wizarding magic demonstrably depletes practitioner reserves through sustained use. Harry exhibits visible exhaustion following extended magical combat, and even powerful wizards like Dumbledore require recovery periods after significant magical exertion. The wand cannot conjure unlimited magical energy.
Complex spells demand proportional magical investment, with the Patronus Charm requiring substantial concentration and energy even for Harry's considerable talent. Extended duelling scenarios favour fresh combatants, and magical fatigue has proven decisive in numerous documented wizarding conflicts.