Looks like you would have to edit includes/functions.php, this section:
PHP Code:
if ($permanent)
{
$expire = TIMENOW + 60 * 60 * 24 * 365;
}
else
{
$expire = 0;
}
which is around line 2158 in the 4.1.8 code. It seems there are only two choices for expiration, either "session", which lasts as long as the browser window stays open, or "permanent", which is coded to 1 year. So if you change it you'll be changing the timeout for any other "permanent" cookies (if there are any).