It depends, really. Many employers just want to see a college degree - it doesn't really matter what it's in - to show that you have the ability to finish what you start; the job may not require any particular academic training. On the other hand, some positions absolutely require such training.
To take a few examples from my own experience:
1) I spent several years teaching middle school math. Although I actually have a minor in mathematics (and have done math research for the last few years), any degree would have done as long as I passed the math PRAXIS or PLACE test; you don't need to know anything too advanced at the middle school level.
2) My last full time job was editing a math textbook and again, while the classes may have gotten me in the door, it was demonstrating the ability to do the job that got me hired.
3) On the other hand, I now work as a software developer and they absolutely wanted to see relevant education; they didn't even care what languages I already knew, they just wanted to know that I understand the basic concepts and have the ability to learn new languages.
So I can't give you a percentage, but I would guess that it's fairly common for people to move into jobs unrelated to their original studies, with the exception of those that require advanced training.
Rambling enough for you?