The behavior you are trying to achieve is not a function of Creo, but of the licensing software Creo uses, which is FlexLM. Having the license manager reclaim licenses that have been IDLE beyond a certain time is discussed here:
IDLE Time Setting
To do this kind of thing you will need to have access to the license server, which might not be the case if you're at a company or strict educational institution.
Also, this is more of a topic for the groups that discuss such matters, maybe the FlexPLM group, or System Administration...
