It's pretty weird the MC never realised the mana he used to power his spells never belonged to him and thus never regenerated. So, he had a finite pool, but had no sense of its limits, using it heedlessly believing it will last forever. Whoever gave him all that mana, without telling him he will run out eventually, with no forewarning of it happening, was quite nasty in that sense. Perhaps magicians in that world usually do sense their own level of remaining mana, like a person could sense their stamina level, knowing if they can still keep running for another kilometer or not, but Ike clearly didn't have any such sense thanks to only containing foreign mana in his body.
He naturally should be grateful he did get this far relying on the externally gained mana, but nonetheless it would have been polite and fair to warn him it will run out one day, so use it extremely sparingly.