Plugin To Disabled the Ninja Form Trash Submission after 30 days.

functions.php

/**
 * Solution to disable the auto cleaning of trash submission in Ninja forms.
 * http://ryansechrest.com/2013/09/prevent-wordpress-automatically-purging-trash-custom-post-type/
 *
 * @param $post_id
 */
function do_not_schedule_post_types( $post_id ) {

	// array of post_type you don't want to be scheduled
	$unscheduled_post_types = array( 'nf_sub');

	if ( in_array( get_post_type( $post_id ), $unscheduled_post_types ) ) {
		delete_post_meta( $post_id, '_wp_trash_meta_time' );
	}
}
add_action ( 'trashed_post', 'do_not_schedule_post_types' );