From http://wordpress.stackexchange.com/questions/24164/redirecting-or-displaying-a-message-on-first-login

redirect-user-on-first-login.php

function my_redirect( $to, $requested, $user ){
    if( !isset( $user->user_login ) ){ // we only want this to run when credentials have been supplied
        return $to;
    }
    $regtime = strtotime($user->user_registered);
    $now = strtotime("now");
    $diff = $now - $regtime;
    $hours = $diff / 60 / 60;
    if( $hours < 48 ){
        return "/somepage/"; // it's been less than 48 hours, redirect to message.
    } else {
        return admin_url();
    }
}
add_filter('login_redirect', 'my_redirect', 10, 3);