If you set a maximum timeout of 600 on the JavaTddPlugin and click "edit", the web interface kicks it back with the error message "The maximum timeout allowed is 600. Contact the administrator for higher limits." Submitting with 599 works.
This bug is more aggravating than it might be because sometimes a new assignment will pre-populate the field with 600, and then turn around and fail when you go to edit the plugin properties.
This was fixed at some point in the past, but the bug was never closed.