By default the session timout is set to the same as the ASP.Net timeout + 10 minutes.

Here is the internal code for it:

            if (!WAFRuntime.Engine.IsSessionTimeoutSet) {
                WAFRuntime.Engine.SessionTimeout = new TimeSpan(0, httpSession.Timeout + 10, 0);
            }

You can set your own by setting this value manually:

            WAFRuntime.Engine.SessionTimeout

or perhaps better, specify it in the web.config by addting this:

<appSettings>
.....
.....
    <add key="WAF.SessionTimeout" value="20" />
.....
.....
<appSettings>

The unit is minutes.

Will this help?

The Webnodes Session uses a "sliding expiration". Each request related to a session will reset the count down.