[abh19]

I don't think there's a way to add a stop date to repeating actions, so you'll have to stop the action manually.

If it's not part of a particular project, you can create a new action in your "Miscellaneous" single actions list. In the inspector, assign the due date to be the 20th, select "Repeat Every" 30 days.

Now add a note to your calendar on 20.10.2010 to remind yourself to delete this action from omnifocus.