Checks the last time plugins were run before checking plugin versions.

This might have been backported to WordPress 2.6.1 for performance reasons.
This is used for the wp-admin to check only so often instead of every page
load.

function _maybe_update_plugins() {
	$current = get_site_transient( 'update_plugins' );

	if ( isset( $current->last_checked )
		&& 12 * HOUR_IN_SECONDS > ( time() - $current->last_checked )
	) {
		return;
	}

	wp_update_plugins();
}