If you're using the latest build of MindManager 2012, you can go to the File menu, select options and then select Task Info. Then, in the task info page, make sure you check the box that says Show Effort Field.
In you map, you can now add effort to tasks (e.g. 4, 9, 12 hours). These hours will roll up with the other task info, if there is any on the topics.