US senators serve 6 year terms.  The elections are staggered, so approximately  1/3 of the Senate seats comes up for election every two years. There is no term limit on the number of times they can run.

http://www.senate.gov/reference/reference_index_subjects/Senators_vrd.htm

US senators serve 6 year terms.  The elections are staggered, so approximately  1/3 of the Senate seats comes up for election every two years. There is no term limit on the number of times they can run.

http://www.senate.gov/reference/reference_index_subjects/Senators_vrd.htm