Senator Daniel K. Inouye was the second-longest serving Senator in history, so the chances of retiring and gathering social security from a life in the Senate are slim.
On the other hand, 16 Senators have been promoted to President, so if you're truly ambitious, those are great odds.
The average tenure for a US Senator is about two terms. That number has risen over time, perhaps because political life has lot more appeal (good food in the cafeteria, free food and drinks at frequent cocktail parties). Another reason why term limits are becoming a hot topic.