Hi,
The max is one month. I don't know of any way to get more than that.
http://www.cisco.com/c/en/us/td/docs/app_ntwk_services/data_center_app_services/application_networking_manager/5-2-3/user/guide/User_Guide/UG_monitor.html#wp1326696
Please visit section "Configuring Historical Trend and Real Time Graphs for Devices" for more details.
Regards,
Kanwal
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