you could imagine the spell as a circuit, and "mana" (or whatever is fuelling it) being electricity; you'd be wasting mana and the spell would be still running>Is magic a finite resource?
there could be two components: the spell itself, and the mana needed to run it; don't know where the ingredients for either or both of them could come, though>Will the spell implode on itself when it runs out of Mana?
it could shut down as if its plug was pulled>If it's a time spell that resets the time in it's AoE will it also reset the amount of Mana, thus being truly infinite and creating a groundhog day pocket dimension?
let's imagine, for example, a character casting a fireball somewhere and then resetting time, then fireball, then reset, etc.
First possibility: the character who reset time went back in time (so if he was stocking mana with himself, or carrying it within himself, it wouldn't be reset, so mana should be external), he would be near the place where his first instance used the fireball, and the amount of available mana in the environment would still be down one fireball's worth of mana.
Second possibility: the character makes the environment go back in time, not himself; the success would depend on whether casting the time travel spell (to bring mana back to the area) consumes less energy than what was available in the area at the moment the time travel speel was cast>And if spells draw Mana from the caster, will that loop kill them?
There could be a failsafe spell, or not, in the latter's case, depending on the story, it could drain the caster's life or cause other side effects like affecting his ability to cast magic.