Its so hard to judge this because it depends on several factors. Some Senators retire after only one or two terms while others stay in until they die on the floor of the Senate Chamber. Also, some may run for Governor or seek their party's presidential nomination. I doubt we'll ever see as many retirements in one year as we did in 1996 because there is so much pressure not to from the party leaders.
That being said, I agree that 2012 seems like a good year for there to be a high turnover rate. Republicans will only have 9 seats to defend and will be looking for some pickups (although the 2012 presidential coattails are yet to be determined). Also, there are 4 Senators that will be over the age of 80 in 2012: Akaka-88, Byrd-95 (DAMN!), Kennedy-80, and Lugar-80. In addition to that, 9 more Senators will have surpassed 70 by 2012: Feinstein-79, Kohl-77, Lieberman-70, Lott-71, Ben Nelson-71, Bill Nelson-70, Sanders-71, and Thomas-79. So nearly 40% of the Senators up for re-election will be over the age of 70. On top of that Sen. Thomas is battling cancer and will likely retire or die
by 2012 and CT liberals will be itching to challenge Sen. Lieberman after 2006 even if he doesn't retire. And one final note on 2012 is that Sen. Brown has said he will not seek re-election but we all know how likely that is.
Having said all that, 2010 could be another year of many new faces. 12 Senators up for re-election will be over the age of 70 (two over 80) in 2010 and three are currently being mentioned as 2008 Presidential candidates: Obama, McCain, and Brownback. In addtion Brownback has said he will not run due to self imposed term limits. The partisan split is fairly even with 15-D and 19-R (this means little though since going into 2006 it was the exact opposite and 6 Republicans lost).
Also, lets not forget that 10 new Senators started their first terms this year. 6 defeated incumbants and 4 took over open seats.
