performanceInsightsRetentionPeriod   
  The number of days to retain Performance Insights data. The default is 7 days. The following values are valid:
- 7 
- month * 31, where month is a number of months from 1-23 
- 731 
For example, the following values are valid:
- 93 (3 months * 31) 
- 341 (11 months * 31) 
- 589 (19 months * 31) 
- 731 
If you specify a retention period such as 94, which isn't a valid value, RDS issues an error.