This is the correct operation for the GtkComboBox. This quote taken from the documentation for the signal.
The changed signal is emitted when the active item is changed. The can be due to the user selecting a different item from the list, or due to a call to gtk_combo_box_set_active_iter(). It will also be emitted while typing into the entry of a combo box with an entry.
There is no way that GTK can know when the user has stopped typing therefore you will need to handle the signal for every character entered and decide yourself when typing has finished.