Assuming you get hired at the Assistant Professor level, you have a six year "probationary" appointment. (That can be extended by a year for health reasons, having a kid, etc.). You should get annual reviews up until that point, and you could get booted earlier, but by your sixth year, you must go up for tenure (your "mandatory" year). Some will go up early, but those are the "stars" and they should fly through the committees (otherwise, they would just be told to wait another year, and get more work published, etc.). If you don't get tenure, then you get the one year terminal appointment like I mentioned before (basically a year to look for a new job).