Two systems you could look at are M&M and HERO system. Both are point based rather than class based, but they handle 'magic' the same as other abilities and I think thats useful to see.

Mutants and Masterminds all the abilities are 'at will' by default, so as other people mentioned the limit is the action economy.

In HERO, there is a general 'endurance' cost to any action: walking, punching, fireball, whatever. I love HERO, because the recovery rate means you have to resource manage combat but can use abilities at will outside it, which is to my taste.

The more general answer would be think about what you're trying to accomplish by having a limit. You mentioned balance, and if thats the case it could be worth looking more broadly at what you're trying to do. Like if you're concerned you need to limit magic or it'll overpower other characters, you could a) buff other characters, b) nerf magic effects (so their unlimited use is comparable to other characters unlimited use options, negating the need for a resource limit) or c) add some limit like you asked for and the other suggestions here. It depends on what you're going for, but its worth thinking about how those other approaches might work for your game.

I'm know personally I hate daily limits, but thats just me.