I used to use startup manager, as pointed out by Chad--24216, but alas, it's no longer being maintained, nor is it in the repositories.
It has been superseded by "grub-customizer", which while up to date, is also not in the standard repositories. To to add and install it simply do the following at the terminal:
sudo add-apt-repository ppa:danielrichter2007/grub-customizer
Press enter to confirm adding the PPA
sudo apt-get update
sudo apt-get install grub-customizer
Press Y to confirm
You can now launch grub-customizer in the usual ways. If you ignore the complete list you'll see when it first launches, and just press "Preferences" button on the toolbar you'll get a nice summary dialog where you can change the timeout value and default menu item, as shown below:Grub Customizer preferences screen