We go back much further to about 3-4 years. Because different emails are sent at different times of year to different audiences with different outcomes, we can't realistically set goals and benchmarks without doing analysis on all three of these factors. If an email is only sent once per year (say, for Father's Day), it's helpful to have that data on hand.
That being said: I wouldn't necessarily store that data in Revenue Explorer. Here, we have hard copies/digital copies of the emails themselves stored on a local srver, and have made historical Excel sheets (for high-level analysis) and queriable databases (for granular analysis) to go further back. Realistically, you probably only need a year's worth of data before looking to export that data to another platform.