Odds of Hanging On
Senator Daniel K. Inouye served in Congress for forty-nine years, and that's not even good enough for the record. Once you actually get elected senator, people generally want to continue electing you. To begin with, you get six years automatically—barring some scandal that gets you kicked out of office.
The average tenure for a U.S. Senator is about two terms (source). That number has risen over time, perhaps because they're getting better at holding onto their jobs. Or at least better at hiding the stuff that gets them kicked out.