Set a session timeout in the file
DIST_ROOT/Src/LS-Client/Default-WebApp/WEB-INF/web.xml of the web client of LibreSource.
Then recompile, just by calling
./ThirdPart/ant/bin/ant at the DIST_ROOT directory.
The number is in minutes.
<web-app>
<session-config>
<session-timeout>60</session-timeout>
</session-config> …
</web-app>