12 years for Reps might be alright, but 12 years for Senators would mean 2 terms, that's it. Sure, it's the same as the president, but the president is more influential, and doesn't have a fraction of the power of one branch, but the whole power of one branch.
The reason that there aren't term limits for Representatives and Senators is because they simply don't have as much power. I'll remind everyone that term limits for presidents are less than 100 years old, constitutionally speaking. If someone is really good at their job as a senator, why limit them? Presidentially speaking it makes more sense, because the balance of power could shift enough in 8 years to do some serious damage, but for one member of congress? Doubtful.