Reports are stored on the Junos Space server where Network Director is running; however, you set the report retention time in Network Director. The default for the report retention period is 30 days. Because accumulating old reports could eventually impact system performance, you might want to consider changing this setting. The report retention period is set in Preferences on the Report tab.
Another option is to store the report to another location such as a Secure Copy Protocol (SCP) server and then delete the report from the Network Director application server.