I've been having this issue with seemingly random documents... It tends to occur when a document is even slightly complex. (By that I just mean it could have lots of text or vectored images. Nothing more complex than that. Even when the text is entirely outlined.)
Not all documents are like this. Most save just fine. Sometimes Illustrator freezes for a few moments, but yesterday one file took an hour to save. The first time I tried I waited 30 minutes but then force quit the application because I had to move on, but I lost all my work. I had to redo everything again, so the second time I just waited it out.
I tried trashing preferences like someone suggested on another thread here, but that accomplished nothing except me having to completely redo my preferences even though I put them back.
Any other suggestions?
This is what I'm running on:
Note: This could be unrelated but at this point I don't know - the server I connect to for work is very slow right now, and we've been having connectivity problems. I work from my desktop but then copy everything back once I'm done. Could this be affecting my computer?