This question obviously has no simple answer, but I'd boil it down to three things that will help answer in a fairly reliable manner depending on the setting.
First thing, the source of magic. Even if it's not revealed in the story, it helps if you have an idea where magic comes from, because that will dictate many of its limitations. The classic fantasy thing that mana infuses the world - even that is, in a way, an answer to the question; although it's not enough to narrow down much, it's enough to draw some bounds. If you give a more detailed answer, for example that magic is a force responsible for the creation and balance of the world, you'll automatically indirectly say its theoretical limits are virtually nonexistent.
This can go on for a while as we come up with better and more detailed answers, but it won't get us anywhere by itself unless we get ridiculously detailed. That's where two more questions come in.
Second, what kind of being can magic make? It basically asks, what can the most powerful being within the bounds of the system be. It's pretty related to the previous question and often affected by it, but it already gets us closer. If the answer is superhuman, well, then it's just superhuman and you've not much room for choice in the strength of magic. It can be a god-like being, it can be a god (an important distinction there, one that becomes especially important if your magic comes from a god), it can be a transcendental whatever that would mean in your story, there's a lot more room for choice than it seems at first. It can even be a term existing for the sole purpose of showing the heights magic can reach.
Third, how hard it is to reach that level? In other words, how many people on that and similar level do you want to have in your story. If you want none, if you want the pinnacle of magic to be some distant dream of the foolish mortals, it means magic's got to be hard, or privileged. If you want pinnacle of magic to be something known that people are trying to go past, it means the magic's easier.
I think, if you answer there three questions, you can get a really good idea where magic stands in your story, and it'll help with consistency. Unless someone decides that breaking the rules they set themselves is a good idea, they'll be forced to write within (hopefully) believable and internally logical bounds and find good ideas there. And as long as that's there - internal logic and consistency - then I can accept virtually any magic system.
Edit: Also, if answering those broadly doesn't help, it can be a good idea to narrow down only to a place or something. For example, even if your magic is theoretically omnipotent, you can say that there's not enough magic on Earth/somewhere to reach a higher than certain level.