Send email to user if not logged after 3 weeks

alter-login.php

<?php

/**
 * Plugin Name:       Alert Last Login
 * Plugin URI:        
 * Description:       
 * Version:           1.0.0
 * Author:            Codeat
 * Author URI:        http://codeat.it
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 */
function user_last_login( $user_login, $user ) {
	update_user_meta( $user->ID, '_last_login', time() );
}

register_activation_hook( __FILE__, 'last_login_activate' );
register_deactivation_hook( __FILE__, 'last_login_deactivate' );

function last_login_activate() {
	//Use wp_next_scheduled to check if the event is already scheduled
	$timestamp = wp_next_scheduled( 'check_users_last_login' );

	//If $timestamp == false schedule
	if ( $timestamp == false ) {
		wp_schedule_event( time(), 'daily', 'check_users_last_login' );
	}
}

function last_login_deactivate() {
	wp_clear_scheduled_hook( 'check_users_last_login' );
}

function execute_plugin() {
	add_action( 'wp_login', 'user_last_login', 10, 2 );
	add_action( 'check_users_last_login', 'send_mail_login' );
	//send_mail_login();
}

add_action( 'plugins_loaded', 'execute_plugin' );

function send_mail_login() {
	$months_ago = strtotime( '-3 weeks', time() );
	echo $months_ago . '-' . time();
	$users = get_users( array(
		'meta_key' => '_last_login',
		'meta_value' => $months_ago,
		'meta_compare' => '<',
			) );

	foreach ( $users as $key => $user ) {
		$user = $user->data;
		wp_mail( 'admin@localhost.dev', 'User '.$user->user_login.' not logged since 3 weeks', 'Visit our website ' . $user->user_login );
	}
}