There are two problems I have with Congressional term limits.
First, too often it seems like it's a matter of people objecting to someone else's representative or senator. For example, people in Mississippi may think Ted Kennedy's been there for too long (going on 46 years) but have no problem with reelecting Thad Cochran (going on 30 years).
Second, and this is the real biggie -- any term limits that would pass Congress would grandfather in the incumbents. In other words, term limits on all future members but not on the bums we'd like to see go. For term limits to have any real change they would have to be retroactive.
I would say 5 or 6 terms for US House members and 2 terms or 15 years max for Senators (similar to the 2 terms/10 years max limit for presidents). Heck, I'd even say repeal the 17th amendment and allow the individual states to decide how to select their senators. If some want to have the state legislatures choose them, then let them. If anything, that would help give more focus on the state legislature elections.
A couple of little tidbits of trivia --
Here's a list of US Senators by seniority. 31 Senators have been in office for over 18 years. That means they have been in office longer than a first time voter this year has been alive. 43 have been in office over 12 years.
Here's a list of US House of Representatives by seniority. 71 (16%) have been in office over 18 years, 166 (38%) over 12 years, and 214 (49%) over 10 years.