public long getGlobalSessionTimeout() {
        return this.globalSessionTimeout;
    }
Returns the system-wide default time in milliseconds that any session may remain idle before expiring. This
value is the main default for all sessions and may be overridden on a <em>per-session</em> basis by calling
{@code Subject.getSession().}{@link Session#setTimeout setTimeout(long)} if so desired.
<ul>
<li>A negative return value means sessions never expire.</li>
<li>A non-negative return value (0 or greater) means session timeout will occur as expected.</li>
</ul>
<p/>
Unless overridden via the {@link #setGlobalSessionTimeout} method, the default value is
{@link #DEFAULT_GLOBAL_SESSION_TIMEOUT}.

@return the time in milliseconds that any session may remain idle before expiring.
core/src/main/java/org/apache/shiro/session/mgt/AbstractSessionManager.java