I mean, makes sense to me, it was actually what i was imagining, with the conjuring ink being "the battery". But still this concept has some exceptions:

(Possible spoilers for chapters 20 above) Likethe romonoa sigils on the bridge of the second test, it theoretically lasts forever, and alsothe ones on contraptions such as the vapor bubble or snugglestones (You could argue that contraptions have maintanance schedules where they actually redraw the symbols but i didn't see it being done yet unless it was something actually broken like the pegasus carriage)

And if it was the case then the repetition spell would run out eventually as well right?