Now I have the information I need. The correct patch is:

--- a/django/forms/fields.py
+++ b/django/forms/fields.py
@@ -492,7 +492,7 @@ class DurationField(Field):
 
     default_error_messages = {
-        'invalid': _('Enter a valid duration. The format is: [DD] [HH:[MM:]]ss[.uuuuuu].'),
+        'invalid': _('Enter a valid duration. The format is: [DD] [[HH:]MM:]ss[.uuuuuu].'),
     }