Index: wp-includes/load.php
===================================================================
--- wp-includes/load.php	(revision 18776)
+++ wp-includes/load.php	(working copy)
@@ -151,8 +151,10 @@
 
 	include( ABSPATH . '.maintenance' );
 	// If the $upgrading timestamp is older than 10 minutes, don't die.
-	if ( ( time() - $upgrading ) >= 600 )
+	if ( ( time() - $upgrading ) >= apply_filters( 'wp_maintenance_expiration', 300 ) ) {
+		@unlink( ABSPATH . '.maintenance' );
 		return;
+	}
 
 	if ( file_exists( WP_CONTENT_DIR . '/maintenance.php' ) ) {
 		require_once( WP_CONTENT_DIR . '/maintenance.php' );