The documentation covers this, here's a link to the relevant information: Specifying License Timeouts
Specifically there is a section that talks about the TIMEOUT option:
Timing Out Idle Licenses
To have products return their licenses to the license pool when they are idle, add a TIMEOUT line to the options file.
If your product supports TIMEOUT, you can set a maximum amount of time (in seconds) that a license can remain inactive.
TIMEOUT feature seconds
I hope that this helps you.