Remember, kids: Loop around while flying in the form of a foot-tall fairy in moderation.
So, yeah, if you were worried about Nanase not having access to magic for another few months in comic time and several centuries from our perspective, let this comic put your fears to rest. Nanase was already on the verge of a burnout when she first used her guardian form, hence the extreme circumstances. In this instance, she barely even used her own energy to use it after months of, at least hypothetically, storing extra energy. It's a safe bet the loss of magic was for the minimum one-day duration and not a second longer.
Of course, this begs the obvious question: WHY IS HER HAIR STILL TWO COLORS?! For the answer to that question, stay tuned for the next comic! Yes, again. Yes, I'm aware it's a Friday comic. Just sit there and wait for it. The weekend will just fly by, I assure you.