Final answer:
One-third of U.S. Senators are elected every two years, and a Senator's term lasts for six years.
Step-by-step explanation:
The fraction of U.S. Senators that is elected every two years is one-third. This is because the Senate is designed with staggered terms to ensure continuity. Therefore, every two years, approximately 33 or 34 seats are up for election. Senator terms last for six years, which allows them to focus on legislation and policy with a longer-term perspective compared to members of the House of Representatives, who serve two-year terms and must run for election more frequently.