We might assume that Eve is typing in passwords, or is a massively parallel computer. Assume that Eve is modeled by a finite collection of deterministic Turing machines. Normally we're very generous with our assumptions of what the attacker can do. If we limit ourselves to more conservative resources we get numbers between 2 exp 150-250 operations. It will turn out some keys have relatively dense vs. relatively sparse keyspaces.
Many ciphers have significantly smaller keyspaces.
Usually that's an artifact of designers assuming that the model is different from what it turns out to be. Every 18 months you lose a bit according to Moore's law. Anything she wants. Yes, with assumptions that are difficult to achieve in practice. Not a fancy cipher.