Congressional term limits should be set at 18 years for the House (i.e., 9 terms) and 18 years for the Senate (i.e., 3 terms). Please see the argument as to the reasons for the length. This Wikipedia article may be useful for background information on term limits generally (including a history of the last attempt to impose them in 1994, when congressional Republicans tried to get them passed but fell short of the 2/3 requirement for a Constitutional amendment): http://en.wikipedia.org/wiki/Term_limits_in_the_United_States
Congressional term limits should be longer than for the executive in order to create stability and provide time to develop expertise in complex matters, especially when the regulated tend to be life-long members of their own fields.
A longer term also makes the idea more palatable to those currently voting on it.
Term limits don't prevent the concentration of too much power. They exacerbate that problem by transferring the actual power to the unelected (subject matter expert) lobbyists. Expertise in process and in subject matter become under-valued; the quality of representation does not improve. It also effectively removes the decisions about representation from the voters.