UNTESTED: Check if a user is already at level 1. If they have been at that level for 30 days, upgrade them to level 2.

my_pmpro_check_and_upgrade_level_on_login.php

<?php
/**
 * UNTESTED: Check if a user is already at level 1. If they have been at that level for 30 days, upgrade them to level 2.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_check_and_upgrade_level_on_login( $user_login, $user ) {
	// Set the membership levels
	$level_1 = 1; // Level 1 ID
	$level_2 = 2; // Level 2 ID

	$user_id = $user->ID;

	// Get current membership levels for the user
	$current_levels = pmpro_getMembershipLevelsForUser( $user_id );

	// Check if the user is at level 1
	if ( isset( $current_levels[ $level_1 ] ) ) { 
		// Check if the user has been at level 1 for 30 days
		$startdate = $current_levels[ $level_1 ]->startdate;

		// Calculate the time difference in days
		$days_at_level_1 = ( current_time( 'timestamp' ) - strtotime( $startdate ) ) / DAY_IN_SECONDS;

		if ( $days_at_level_1 >= 30 ) {
			pmpro_changeMembershipLevel( $level_2, $user_id );
		}
	}
}
add_action( 'wp_login', 'my_pmpro_check_and_upgrade_level_on_login', 10, 2 );