Managing PHP sessions and timeouts

I am a beginning PHP programmer with a lot of Classic ASP experience. I am creating a new web app where users login and the login status is controlled with a PHP session. I have this bit of code inserted at the beginning of all my pages, via an include file:

session_start();

if (!isset($_SESSION["user_id"])){
	$user_login=0;
} else {
	if ($_SESSION["user_id"]==""){
		$user_login=0;
	} else {
		$user_login=1;
	}
}

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For pages where I don't want non-logged in users to have access, I insert this at the top:

if ($user_login==0){
	header("Location: default.php", true, 303);
	die();
}

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That simply bounces them back to the home (login) page if $user_login==0, rather than loading the rest of the page. If they are logged in ($user_login==1), then the page will load.

When the user clicks a "log out" button, I simply call this:

$_SESSION["user_id"]="";

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All of that works fine. The odd thing is that my sessions don't seem to timeout. It is my understanding that a PHP session will timeout after 20 minutes of inactivity (similar to an ASP session). However, even after letting my browser window sit idle for over an hour, if I hit "reload", the browser does not redirect backt to the home (login) page.

Is there something I am missing? Also, is the way I am coding sessions in the above examples pretty solid, as far as PHP goes? Would appreciate any advice or critique on what I'm doing.

Thank you.