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.