David Karol has written this article in Congress and the Presidency.  Here is the abstract:

Congressional retirement decisions affect representation and campaigns. They have long interested scholars. Yet the timing of retirement announcements has not been explored. In an analysis dating to 1920 based on an original data set, I show that U.S senators now announce their retirements far earlier in their final term than they once did. Beyond documenting this little-noted trend, I propose and assess explanations for these findings, focusing on changes in campaign finance law. I discuss implications for representation and the growth of the “permanent campaign.”