Hi,
We actually extended the time to a week to account for extended weekend plus some
introscope.enterprisemanager.autoUnmountDelayInMinutes=10080
introscope.enterprisemanager.agentconnection.metrics.agentTimeoutInMinutes=10080
My original setting was 3 days, but Operations wanted more.
I had the same question and at the time (v9.5) it was said not to be too concerned about the performance drawbacks. The EM tracks the agent in case the interruption was temporary and the first property controls the delay before it releases the resources associated with tracking the agents.
The only side effect is that if the agent is down for a week and if you restart the MoM or modify the module with the alert setting, the alerts are re-evaluated and resent. I do not remember the exact case, but one of the alert I set is triggered when it falls to 0 and so I think I get the alert when the agent unmounts 5 days later. If you have lots of agents and a certain percentage in maintenance it is a bit of nuisances and forced us to plan the MoM restarts.
Regards,
Fred