Check the "Computers" tab, to see what machine is grabbing the license. If the machine has not any 3dsmax session opened, check for background 3dsmax.exe processes that could be still running or hanged. Check also for antivirus or internet security programs that could be blocking the access to the license server.
If 3dsmax is shuttled down properly the license is freed immediately, so should be not necessary to change the timeout settings. However you can change the default settings in this registry key:
64 bit systems:
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Itoo Software\License Server]
32 bit systems:
[HKEY_LOCAL_MACHINE\SOFTWARE\Itoo Software\License Server]