Key length determines how many different keys you can generate. A keylength of 56 bits means you can have 2^56 keys or 72057594037927936 different keys. Given todays computing power, that's considered a weak keylength.

The complexity of the key shouldn't matter ... let me say that again ... shouldn't matter ... because your key needs to change on a periodic (or at least aperiodic) basis. Not changing your key on a regular basis gives your attacker more data to work with, more time to do it in, and exposes more data to potential loss.

That being said, a 56 bit key that never changes may be just fine for you. It's better than XOR but nowhere near as good as AES (however, the longer you keep the same key, the more it becomes like XOR). It all depends on the value of what your protecting and what other layers of security you have.

Does that answer your question? Probably not. The important thing to remember is key complexity is a red herring because your key should be changing and 56 bit encryption can be good enough if you have other layers of security.