Yes, you are correct!
Historical trending in Salesforce is subject to certain limits on the time during which data is tracked, the number of rows of data, and the fields and objects you can track. These limits are designed to restrict the data set so that reports return data quickly.
You can make up to three months of data available for historical trend
Salesforce retains historical data for the previous three months, plus
the current month.
This comes under the Limitations on Historical Trend Reporting.
When I enabled this feature in my developer org, I have received the following email from Salesforce.com
A new custom report type, Case with Historical Trending, is available.
This report type can compare data for up to five points in time
(snapshots) within the previous three full months and all days so far
in the current month.
Space is available for up to 5 million records. You are alerted by
email if you reach 70% of the limit. To avoid hitting the limit, use
the Historical Trending setup page to configure the amount of data for
which history is captured.