The Web Page Security Validation feature helps enhance security by
imposing a time limit on Web pages when a user submits information to
the server. When a user tries to submit information to the server
after the validation time-out expires, the user receives the error
message that is mentioned in the "The security validation for this
page has timed out." section.https://support.microsoft.com/en-us/kb/888828?wa=wsignin1.0
If all of that failed, why not just add Javascript to the master page that uses setTimeOut() to redirect the user to the logout page after 20 minutes? By default, this page is at /_layouts/SignOut.aspx but your FBA may have specified a different one.
Source: Automatic Logout the User after "X" time in Sharepoint 2013
Here is the script: How to: enable inactivity log out for SharePoint server
You have to use code in master page.
$(document).ready(function () {
idleTime = 0;
LogOutInactive();
});
function LogOutInactive() {
var idleInterval = setInterval(timerIncrement, 60000);
$(this).mousemove(function (e) {
idleTime = 0;
});
$(this).keypress(function (e) {
idleTime = 0;
});
}
function timerIncrement() {
idleTime = idleTime + 1;
if (idleTime > 9) {
window.location = "http://[your site]/_layouts/15/SignOut.aspx";
}
}