Answer by
mem4dr (1378)

Each term lasts six years. Every two years, 1/3 of the Senate must be re-elected. For example, if group A of Senators is up for re-election, 2 years later another 1/3 of Senators faces re-election, 2 years later the final 1/3 faces re-election, and 2 years after that (6 years after group A was elected), group A faces re-election again.