Index: wp-admin/includes/ajax-actions.php
===================================================================
--- wp-admin/includes/ajax-actions.php	(revision 24706)
+++ wp-admin/includes/ajax-actions.php	(working copy)
@@ -2103,9 +2103,9 @@ function wp_ajax_get_revision_diffs() {
 		wp_send_json_error();
 
 	$return = array();
-	@set_time_limit( count( $_REQUEST['compare'] ) );
 
 	foreach ( $_REQUEST['compare'] as $compare_key ) {
+		@set_time_limit( 0 );
 		list( $compare_from, $compare_to ) = explode( ':', $compare_key ); // from:to
 
 		$return[] = array(