Yes this time in configurable changing web.xml file ( Into OpenKM.ear/OpenKM.war/Web-inf/web.xml).
Take a look at this
url.
You need to add something like this:
<session-config>
<session-timeout>30</session-timeout>
</session-config>
I don't recomend decrease session timeout less than 5 minutes,if you've got in on mind, because every 5 minutes OpenKM UI, fires keep alive to maintain session active, if during 5 minutes you don't do any operation on OpenKM and sets a lower time out probably you'll lose more sessions than you want.
Really on administration there's a action to logout users for this purpose, use it. I don't recomend decreasing session time out.