It is great to be able to set the maximum length of the recording in the GUI, too.
This is the patch that allows you to do just that.
Find the corresponding preference under the "Misc" tab.
The patch is included.
It only works if the corresponding patch is applied to the the recordmydesktop command-line tool.
It would be great if this made its way into the official version.
P.S. I didn't do this for Qt, since I have no experience with it.