Analytics data of dashboards are retained for a year and custom reports data retained for 6 months.
@Karthik According to the notes section of Analytics docs page:
Data older than six months from the current date is not accessible by default. If you want to access data older than six months, contact Apigee Support.
Same is stated on the custom analytics page.
Just want to add a comment here as since this original post the retention policy has changed and folks are still seeing this posting. Retention now differs based on Edge License. Starter is 90 days but it is 12 months retention for higher-level licenses,
Here's the Spec sheet list
Here's the Spec sheet for Edge Cloud