cron-delete-revisions.php

<?php
	// add filter to allow for new weekly schedule
	add_filter( 'cron_schedules', 'rkv_weekly_cron' );

	function rkv_weekly_cron( $schedules ) {
		// Adds once weekly to the existing schedules.
		$schedules['weekly'] = array(
			'interval' => 604800,
			'display' => __( 'Once Weekly' )
		);
		return $schedules;
	}

	add_action( 'rkv_revision_cron', 'rkv_revision_delete' );

	// set cron schedule
	function rkv_revision_schedule() {
		if ( !wp_next_scheduled( 'rkv_revision_cron' ) )
			wp_schedule_event( time(), 'weekly', 'rkv_revision_cron' );
	}
	add_action( 'wp', 'rkv_revision_schedule' );

	// run deletion on revisions
	function rkv_revision_delete() {

		// query revisions and get array of IDs
		$args = array(
			'fields'        => 'ids',
			'post_type'     => 'revision',
			'numberposts'   => -1
		);

		$revisions = get_posts( $args );

		// loop through and delete each
		foreach ( $revisions as $revision )
			wp_delete_post( $revision );
	}