Unfortunately,  there is not a native way to automatically remove agents from the VSA after X number of days.

However, you can create a View that shows agents that haven’t checked in for X number of days.  

Like this:

For any agent falling into that view, set it to “Delete without uninstalling”. Try and do this at the end of every month, just before you bill your clients. It helps with billing overages because the client failed to report a retired computer.  By using the option to Delete without Uninstalling, if the computer comes back online, the agent will check back in.  This can be both a blessing and a curse.  It’s useful for computers that are stored for long periods of time before being put back in use, however, for clients that no longer pay for your services, it can be a bit of a game of “Whack-a-Mole” as the agents pop up from time to time.  Until the agent is actually uninstalled from the computer, either via the VSA, or Add/Remove, it will continue checking in, recreating the agent account if it’s missing, even if the Org no longer exists.